Interandean Depression Research Articles

Case Study of the Application of an Innovative Guide for the Seismic Vulnerability Evaluation of Schools Located in Sangolquí, Interandean Valley in Ecuador

The current study is based on the analysis and adaptation of a Federal Emergency Management Agency guide, FEMA P-1000, from the USA to improve school safety against natural hazards by applying the guide to the infrastructure of Ecuadorian schools, focusing primarily on seismic risk. By considering the technical foundations of structuring and managing disasters in buildings for school use, society will be provided with a practical procedure to recognize those aspects that need immediate attention as part of proper risk management. Here, a variety of parameters are involved in the proposed methodology of the given guide from FEMA combined with the national construction standards and regulations. The characteristics of nearby geological faults and structural and nonstructural vulnerability levels, amongst others, were also considered to allow for a detailed evaluation and a subsequent seismic risk categorization. Finally, the global risk is determined for the studied institutions of Sangolquí in the Valley of Los Chillos, within the Interandean Depression in central Ecuador.

Impact of the Southern Ecuadorian Andes on Oxygen and Hydrogen Isotopes in Precipitation

Determining how the elevation of the Northern Andes has evolved over time is of paramount importance for understanding the response of the Northern Andes to deformational and geodynamic processes and its role as an orographic barrier for atmospheric vapor transport over geologic time. However, a fundamental requirement when using stable isotope data for paleoaltimetry reconstructions is knowledge about the present-day changes of δ18O and δD with elevation (isotopic lapse rate). This study defines the present-day river isotopic lapse rate near the Equator (∼3°S) based on analysis of δ18O and δD of surface waters collected from streams across the Western Cordillera and the Inter-Andean depression in Southern Ecuador. The results for the two domains show a decrease of δ18O with elevation which fits a linear regression with a slope of −0.18‰/100 m (R2= 0.73,n= 83). However, we establish a present-day lapse rate of −0.15‰/100 m for δ18O (R2= 0.88,n= 19) and -1.4‰/100 m for δD (R2= 0.93,n= 19) from water samples collected along the west facing slopes of the Western Ecuadorian Cordillera which is mainly subject to moisture transport from the Pacific. We argue that this empirical relationship, consistent with those obtained in different tropical areas of the world, can inform stable isotope paleoaltimetry reconstructions in tropical latitudes.

Fold-Axis Parallel Extension along the Southern Ending of the Quito (Ecuadorian Andes) Fault System: Implications in River Network and Aquifer Geometry

Extension parallel to fold axis represents a rarely documented tectonic process that occur in different tectonic settings such as large-scale orogenic belts, mountain ranges, and thrust and fold belts. Along the Ecuadorian Andes, extensional systems have only been reported as tectonic boundaries of local intermontane basins, pull-apart depressions and outer-arc stretching faults along hinge surfaces in compressional antiforms. The Quito fault system shows in planform view a right-stepping en-echelon arrangement which southernmost anticlinal ridge terminates as an arcuate structure locally dissected by the Saguanchi Gorge. Valley hillslopes of this transverse drainage display evidence of tectonic extension featured by several swarms of normal faults commonly dipping towards the valley bottom. Magnetotellurics sounding obtained across a N‒S line parallel to the crestline axis reveals three contiguous and distinguishable domains of which the central one shows a wedge-shaped geometry pointing down. This tectonic evidence for ongoing extensional deformation suggests that the process of fold-axis-parallel extension controlled the tectonic development and dissection of the Saguanchi Gorge across the anticlinal ridge. This episode marks a significant change in the groundwater recharge and aquifer storage conditions between the Quito basin and the inter-Andean Depression aquifer systems, currently in production to provide the potable water supply for the Quito city.

New Mesozoic and Cenozoic fossils from Ecuador: Invertebrates, vertebrates, plants, and microfossils

Ecuador is well known for its extensive extant biodiversity, however, its paleobiodiversity is still poorly explored. Here we report seven new Mesozoic and Cenozoic fossil localities from the Pacific coast, inter-Andean depression and Napo basin of Ecuador, including vertebrates, invertebrates, plants, and microfossils. The first of these localities is called El Refugio, located near the small town of Chota, Imbabura Province, from where we report several morphotypes of fossil leaves and a mycetopodid freshwater mussel of the Upper Miocene Chota Formation. A second site is also located near the town of Chota, corresponding to potentially Pleistocene to Holocene lake deposits from which we report the occurrence of leaves and fossil diatoms. A third locality is at the Pacific coast of the country, near Rocafuerte, a town in Esmeraldas Province, from which we report a late Miocene palm leaf. We also report the first partially articulated skull with teeth from a Miocene scombridid (Mackerels) fish from El Cruce locality, and completely preserved seeds from La Pila locality, both sites from Manabí Province. Two late Cretaceous fossil sites from the Napo Province, one near Puerto Napo showing a good record of fossil shrimps and a second near the town of Loreto shows the occurrence of granular amber and small gymnosperms seeds and cuticles. All these new sites and fossils show the high potential of the sedimentary sequences and basins of Ecuador for paleontological studies and for a better understanding of the fossil record of the country and northern South America.

Probabilistic Seismic-Hazard Assessment in Quito, Estimates and Uncertainties

Research Article| October 22, 2014 Probabilistic Seismic‐Hazard Assessment in Quito, Estimates and Uncertainties Céline Beauval; Céline Beauval aISTerre, Univ. Grenoble Alpes‐CNRS‐IRD, F‐38041 Grenoble, Franceceline.beauval@ujf-grenoble.fr Search for other works by this author on: GSW Google Scholar Hugo Yepes; Hugo Yepes bInstituto Geofisico, Escuela Politécnica Nacional, Ladrón de Guevara, 2759 Quito, Ecuador Search for other works by this author on: GSW Google Scholar Laurence Audin; Laurence Audin aISTerre, Univ. Grenoble Alpes‐CNRS‐IRD, F‐38041 Grenoble, Franceceline.beauval@ujf-grenoble.fr Search for other works by this author on: GSW Google Scholar Alexandra Alvarado; Alexandra Alvarado bInstituto Geofisico, Escuela Politécnica Nacional, Ladrón de Guevara, 2759 Quito, Ecuador Search for other works by this author on: GSW Google Scholar Jean‐Mathieu Nocquet; Jean‐Mathieu Nocquet cGeoazur, IRD, Université de Nice Sophia‐Antipolis, Observatoire de la Côte d’Azur, CNRS, 250, Rue A. Einstein, 06560 Valbonne, France Search for other works by this author on: GSW Google Scholar Damiano Monelli; Damiano Monelli dGEM Hazard Team, SED‐ETHZ, Sonneggstrasse 5, 8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Laurentiu Danciu Laurentiu Danciu dGEM Hazard Team, SED‐ETHZ, Sonneggstrasse 5, 8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2014) 85 (6): 1316–1327. https://doi.org/10.1785/0220140036 Article history first online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Céline Beauval, Hugo Yepes, Laurence Audin, Alexandra Alvarado, Jean‐Mathieu Nocquet, Damiano Monelli, Laurentiu Danciu; Probabilistic Seismic‐Hazard Assessment in Quito, Estimates and Uncertainties. Seismological Research Letters 2014;; 85 (6): 1316–1327. doi: https://doi.org/10.1785/0220140036 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The present study is focused on estimating the probabilistic seismic hazard for the capital city of Ecuador, Quito, the population of which currently exceeds 2 million inhabitants at present. Quito is located at 2800 meters above sea level within the Interandean Depression, bounded by the equatorial line to the north, in an earthquake‐prone environment (Chatelain et al., 1999; Fig. 1). The city and its suburbs have developed in a piggy‐back basin on the hanging wall of a reverse fault system (Fig. 2) that has been recognized as seismically active in historical, geomorphologic, geologic, and geodetic... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Active tectonics in Quito, Ecuador, assessed by geomorphological studies, GPS data, and crustal seismicity

AbstractThe Quito Fault System (QFS) extends over 60 km along the Interandean Depression in northern Ecuador. Multidisciplinary studies support an interpretation in which two major contemporaneous fault systems affect Quaternary volcanoclastic deposits. Hanging paleovalleys and disruption of drainage networks attest to ongoing crustal deformation and uplift in this region, further confirmed by 15 years of GPS measurements and seismicity. The resulting new kinematic model emphasizes the role of the N‐S segmented, en echelon eastward migrating Quito Fault System (QFS). Northeast of this major tectonic feature, the strike‐slip Guayllabamba Fault System (GFS) aids the eastward transfer of the regional strain toward Colombia. These two tectonic fault systems are active, and the local focal mechanisms are consistent with the direction of relative GPS velocities and the regional stress tensor. Among active features, inherited N‐S direction sutures appear to play a role in confining the active deformation in the Interandean Depression. The most frontal of the Quito faults formed at the tip of a blind thrust, dipping 40°W, is most probably connected at depth to inactive suture to the west. A new GPS data set indicates active shortening rates for Quito blind thrust of up to 4 mm/yr, which decreases northward along the fold system as it connects to the strike‐slip Guayllabamba Fault System. The proximity of these structures to the densely populated Quito region highlights the need for additional tectonic studies in these regions of Ecuador to generate further hazard assessments.

A Review on Tectonic Record of Strain Buildup and Stress Release across the Andean Forearc along the Gulf of Guayaquil-Tumbes Basin (GGTB) near Ecuador-Peru Border

Gravimetric and geologic data show that the reactivation of the Neogene Interandean depression and/or the ~75 - 65 Ma ophiolite suture into the modern dynamic of the Andes controlled the Gulf of Guayaquil Tumbes basin (GGTB) location and evolution during the past 1.8 - 1.6 Myr at least. Depending on whether the remobilization occurred along the interandean depression or the ophiolite suture, the GGTB evolved trough pure or simple shear mechanisms, respectively. Because the GGTB exhibits an along strike tectonic asymmetry associated with a pervasive seismic gap, the simple shear solution is more likely. Tectonic inversion occurred along a mid-crust detachment (the Mid-Crust detachment hereafter) matching the ophiolite suture that accommodates the North Andean Block (NAB) northward drift. The so-called Decoupling Strip located at the shelf slope break accommodated the tensional stress rotation from N-S along the shelf area i.e. NAB-drift induced to E-W along the continental margin i.e. subduction-erosion-induced. The landward dipping Woollard detachment system located at the Upper-Lower slope boundary connects the subduction channel at depth, allowing the Upper slope to evolve independently from the Lower slope wedge. The long-term recurrence interval between earthquakes, the strong interplate coupling, and the aseismic creeping deformation acting along the main low-angle detachments i.e. the Woollard and the Mid-Crust detachments may account for the pervasive seismic gap at the GGTB area. Because the subduction channel exhibits no record of significant seismic activity, no evidence exists to establish a link between the GGTB sustained subsidence and a basin-centered asperity. Because the GGTB is a promising site of hydrocarbon resources, to understand processes at the origin of this escape-induced forearc basin has a major economic interest.

Adakite-like volcanism of Ecuador: lower crust magmatic evolution and recycling

In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc), and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb) data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center, rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes are small and their values are overall rather primitive (e.g., eNd = +1.5 to +6, 87Sr/86Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation. We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts, rather than of slab melts and slab melt–mantle interactions. The isotopic signatures of the least evolved adakite-like rocks of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic Carnegie Ridge.

The Chota basin and its significance for the inception and tectonic setting of the inter-Andean depression in Ecuador

The inter-Andean structure is defined as an approximately north–south-trending, linear, topographic depression in Ecuador located between the Cordillera Real and the Cordillera Occidental. The depression swings westward toward the Gulf of Guayaquil in southern Ecuador, dissecting the topography of the Cordillera Occidental. The structural limits of the depression are reactivated faults, which formed during accretionary events after 140 Ma along the Ecuadorian continental margin. Several distinct basins progressively formed along the depression. A new, radiometric-based, chronostratigraphic framework for the sedimentary series of the Chota basin has been combined with data from other subbasins in the inter-Andean depression to reevaluate the timing and formation of the larger scale tectonic structures. Inception of the Chota basin commenced at ∼6–5 Ma (latest Miocene), and the progressively younger Quito-San Antonio-Guayllabamba, Ambato-Latacunga, and Riobamba-Alausí basins formed along a southward trend. Each basin was filled with alluvial fan, fluvial, lacustrine, and contemporaneous volcanic deposits. Synsedimentary transpressive deformation was dominant during basin development, though minor, synsedimentary, normal faulting is assumed to have occurred during short periods of weakening compression. The inter-Andean depression of Ecuador formed in the vicinity of a major restraining bend, which accommodates the northward displacement of the north Andean block with respect to the South American plate. A ramp basin model is proposed to explain the tectonosedimentary development of the inter-Andean depression.

Thermochronology of allochthonous terranes in Ecuador: Unravelling the accretionary and post-accretionary history of the Northern Andes

The western cordilleras of the Northern Andes (north of 5°S) are constructed from allochthonous terranes floored by oceanic crust. We present 40Ar/ 39Ar and fission-track data from the Cordillera Occidental and Amotape Complex of Ecuador that probably constrain the time of terrane collision and post-accretionary tectonism in the western Andes. The data record cooling rates of 80–2 °C/my from temperatures of ∼540 °C, during 85 to 60 Ma, in a highly tectonised mélange (Pujilí unit) at the continent–ocean suture and in the northern Amotape Complex. The rates were highest during 85–80 Ma and decelerated towards 60 Ma. Cooling was a consequence of exhumation of the continental margin, which probably occurred in response to the accretion of the presently juxtaposing Pallatanga Terrane. The northern Amotape Complex and the Pujilí unit may have formed part of a single, regional scale, tectonic mélange that started to develop at ~85 Ma, part of which currently comprises the basement of the Interandean Depression. Cooling and rotation in the allochthonous, continental, Amotape Complex and along parts of the continent–ocean suture during 43–29 Ma, record the second accretionary phase, during which the Macuchi Island Arc system collided with the Pallatanga Terrane. Distinct periods of regional scale cooling in the Cordillera Occidental at ∼13 and ∼9 Ma were synchronous with exhumation in the Cordillera Real and were probably driven by the collision of the Carnegie Ridge with the Ecuador Trench. Finally, late Miocene–Pliocene reactivation of the Chimbo–Toachi Shear Zone was coincident with the formation of the oldest basins in the Interandean Depression and probably formed part of a transcurrent or thrust system that was responsible for the inception and subsequent growth of the valley since ∼6 Ma.

(U–Th)/He thermochronometric constraints on the late Miocene–Pliocene tectonic development of the northern Cordillera Real and the Interandean Depression, Ecuador

The low sensitivity of apatite fission track (AFT) thermochronometry at temperatures less than ∼60 °C suggests that AFT data sets from the Andean Cordilleras may have frequently failed to identify specific periods after 9 Ma when cooling rates were high. Forward modeling of (U–Th)/He apatite age data obtained from the juxtaposed Paleozoic–Mesozoic Alao, Loja, and Salado terranes in the northern Cordillera Real, Ecuador, has improved the resolution of previous AFT thermal histories for the past 9 My. The Alao and Loja terranes form a coherent, structural block that resided at temperatures greater than 70–80 °C until ∼3.3–2.8 Ma and then cooled rapidly to less than 40 °C at rates of >15 °C/My. Intraterrane variations in the cooling and exhumation histories in the Salado terrane suggest that nonterrane-bounding faults played a significant role during its Pliocene–Recent evolution. The Salado terrane preserves an older history that reveals elevated cooling rates during 22–19 and 18–15 Ma. Subsequently, the terrane cooled rapidly from greater than 90 °C to less than 40 °C during 11–8 and 5.5–3.5 Ma at rates of >8 °C/My. Vertical reactivation of the Llanganates fault, which separates the Salado and Loja terranes, during the Pliocene–Recent coincides with the main stages of formation of the juxtaposed Interandean Depression, which provides further constraints on the growth phases of the depression and the Cordillera.

The use of the linear reservoir concept to quantify the impact of changes in land use on the hydrology of catchments in the Andes

Abstract. The high Andes region of South Ecuador (The Páramo) is characterised by a cold and wet climate. Most soils of the Páramo region are Andosols and Histosols, with a very high water retention capacity that is affected irreversibly by drying. This key property of Páramo soils buffers catchment outflow, resulting in an almost uniform outflow pattern which, notwithstanding the variability in rainfall, can be very variable in space and time. These soils serve as the most important reservoir of drinking and irrigation water for the densely populated inter-Andean depression region. The Páramo has long served only as an extensive grazing area but recent population pressure and land scarcity have increased cultivation. Two small Páramo catchments (about 2 km2) were monitored intensively for precipitation and discharge for over a year to assess the effect of such land-use changes on the hydrological properties. One catchment is in an undisturbed area and grazed intensively while in the other, local farmers started intensive drainage for cultivation of potatoes about five years ago. The linear reservoir concept has been used to assess the overall retention capacity of the catchments in terms of both peak response and base flow. In this model, every catchment is considered as a series of independent parallel reservoirs, each characterised by mean residence times (T). In every catchment, three major mean residence times can be distinguished. In the undisturbed catchment, an immediate response, characterised by a T of 5.4 hours, is followed by a slower response with a T of 44.3 h. The base flow has a mean T value of 360 h. The response of the cultivated catchment is similar with T values of 3.6 h, 27.2 h and 175 h, respectively. As a result, in the disturbed catchment, water release is about 40% faster than in the undisturbed catchment, so that the base flow falls rapidly to lower levels. The linear reservoir model is a simple way of quantifying the impact of land use changes on the water regulation capacity of Páramo catchments. Keywords: linear reservoir, Páramo, mountain hydrology, land use changes, Ecuador

Deer fauna from Pleistocene and Holocene localities of Ecuador (South America)

Late Pleistocene and Holocene deer remains from Ecuadorian sites have been analysed. Most of the material belongs to Odocoileus virginianus ustus, which is documented in latest Pleistocene deposits, in the Interandean Depression. Coastal sediments referred to the Holocene bear remains of Odocoileus salinae. O. v. ustus and O. salinae differ in size and in the morphology of the antlers and of the dentitions. In the present work, O. salinae is postulated to have derived from O. v. ustus during an arid climatic phase in the latest Pleistocene.

Geomorphic and tectonic evolution of the Ecuadorian Andes

In Early Miocene times the Cordillera did not exist and the Pacific Ocean reached the Oriente. In the Middle Miocene, the uplift of an elongated swell, consisting of Palaeozoic and older rocks, created the Eastern Cordillera. Decollements were activated diverging away from the Eastern Cordillera. In the eastern trench, located approximately in correspondence with the present day Interandean Depression, many thousands of meters of sediments were deposited. A planation surface was created at the end of Lower Pliocene from the Costa to the Oriente graded to sea level. Later, ignimbric flows covered much of the planation surface. Uplift brought the planation surface to 3500–4000 m. The Interandean Depression, bounded by normal faults, was created during the Upper Pliocene, and large strato-volcanoes erupted at this time. The volcanic activity contributed to the filling of an accumulation plateau preserved today in many parts of the Interandean Depression. The creation of the Interandean Depression is the result of lateral spreading activated as a consequence of uplift of the Cordillera. In the Middle and Late Pleistocene the Cordillera and the Accumulation plateau were mostly affected by downcutting with minor episodes of accumulation during the cold phases.

The last occurrence of Pleistocene megafauna in the Ecuadorian Andes

The latest Pleistocene—Holocene megafauna extinction is a global event, particularly dramatic in the Americas. In a previous paper the authors hypothesised a scenario for this extinction event in South America, where mastodonts first suffered from the changing climate environment, followed by the mylodonts and equids. These different latest Pleistocene—Holocene megafauna extinction “waves” in Ecuadorian Andes have been dated using 14C methods on material from selected sites in north and central Ecuadorian Interandean Depression. An outline of the physiographic evolution of the Interandean Depression in Ecuador is offered and the stratigraphic setting of the fossiliferous sites is discussed. The present results confirm the author's hypothesis on the megafauna extinction pattern, previously published in terms of relative age. The importance of climatic changes during Last Glacial Maximum at low latitudes is discussed.

Hypothesis on the cause of extinction of the South American mastodonts

Paleontological, geomorphological and sedimentological investigations on the Cangahua Formation in the Interandean depression of Northern and Central Ecuador have provided information on the evolution of the Andean paleoenvironment during the Late Pleistocene. Pyroclastic and windblown sediments were deposited during cold and dry phases of the last glaciation, interrupted many times by the development of forest-steppe and steppe paleosoils during interstadials. An erosional phase which closed the Cangahua sedimentation was followed by the deposition of colluvial sediments, characterized by a high number of minor pedogenetic episodes. The colluviums are confidently referable to the Holocene. The upper part of the Cangahua Formation is rich in mammal fossils and is probably referable to the Last Glacial Maximum. The fossiliferous sequences suggest that mastodonts disappeared before mylodonts and equids. We hypothesize that the increased cold and aridity of the Last Glacial Maximum, which deeply affected the Cordillera, caused the extinction of most of the megafauna and the mastodonts seem to have been the most sensitive to the environmental degradation. The final history of South American mastodonts, represented by Haplomastodon and Stegomastodon, spans the latest Pleistocene and probably the earliest Holocene. Haplomastodon was dispersed in the highlands within the tropical belt and Stegomastodon in plains of the southernmost part of Brazil, in Paraguay, Uraguay, Argentine, central and northern Chile. Both Haplomastodon and Stegomastodon suffered the same negative effects of the Last Glacial Maximum when their habitats underwent intense desertifications under dry and cold conditions. They disappeared in a mosaic way in the course of the latest Pleistocene, the last representatives probably surviving in favorable restricted areas where however the considerably increased selective pressure was in the long run devastating. In our opinion the human impact was not a determinant in causing mastodont extinction.

Geodynamic evolution of the Neogene intermontane Chota basin, Northern Andes of Ecuador

The development of intermontane basins with a thick non-marine sedimentary pile is one of the main characteristics of the Andean geodynamic evolution during the Neogene. The Chota basin is the northernmost Neogene intermontane basin recognized in the Andes of Ecuador. This basin is located in the Interandean Depression and presents a sedimentary fill of continental deposits. The detailed stratigraphic and tectonic analysis of this basin allows for a new subdivision of the sedimentary sequence in four main units with a total minimum thickness of 2400 m. Sequence analysis suggests a cyclic evolution characterized by two megasequences. The first one, represented by the Chota Unit, displays an evolution from proximal to more distal facies (braided alluvial to lacustrine facies) during the opening of the basin. The second one, represented by the Santa Rosa Unit to the west, and the Peñas coloradas and Carpuela Units to the east, displays a thickening-coarsening evolution related with the filling of the basin and with the development of prograding alluvial fan deposits. The structural analysis in the Chota basin shows two main tectonic events. The first event is responsible for the opening of the basin, and is the result of a tensional tectonic period with σ3 along a N130 °E direction. The second event caused the closing of the basin, as a result of a regional compressional regime with a N120 °E trending, which then rotated to an E-W compressive trend. The Chota Basin is interpreted as a Neogene intermontane basin, with a sedimentary fill controlled by a continuous tectonic deformation. The deposits of this basin recorded large scale geodynamic events and are a unique example for the study of the Neogene evolution of the Ecuadorian Northern Andes.

A Pliocene-Quaternary compressional basin in the Interandean Depression, Central Ecuador

SUMMARY The segment of the Interandean Depression of Ecuador between Ambato and Quito is characterized by an uppermost Pliocene-Quaternary basin, which is located between two N-S trending reverse basement faults: the Victoria Fault to the west, and the Pisayambo Fault to the east. The clear evidence of E-W shortening for the early Pleistocene (between 1.85 and 1.21 Ma) favours a compressional basin interpretation. The morphology (river deviations, landslides, folded and flexure structures) demonstrates continuous shortening during the late Quaternary. The late Pliocene-Quaternary shortening reached 3400 f 600 m with a rate of 1.4 f 0.3 mm yr-'. The E-W shortening is kinematically consistent with the current right-lateral reverse motion along the NE-SW trending Pallatanga Fault. The Quito-Ambato zone appears to act as a N-S restraining bend in a system of large right-lateral strike-slip faults. The compressive deformation. which affects the Interandean Depression during the Pliocene is apparently coeval to the beginning subduction of very young oceanic lithosphere north of the Gulf of Guayaquil. The relatively buoyant new crust may have significantly increased the mechanical coupling in the subduction zone from Pliocene to Present.

Late Quaternary kinematics of the Pallatanga strike-slip fault (Central Ecuador) from topographic measurements of displaced morphological features

The northeast-trending Pallatanga right-lateral strike-slip fault runs across the Western Cordillera connecting N50°E-N70°E trending normal faults in the Gulf of Guayaquil with N-S reverse faults in the Interandean Depression. Over most of its length, the fault trace has been partly obscured by erosional processes and can be inferred in the topography only at the large scale. Only the northern fault segment, which follows the upper Rio Pangor valley at elevations above 3600 m, is prominent in the morphology. Valleys and ridges cut and offset by the fault provide an outstanding record of right-lateral cumulative fault displacement. The fault geometry and kinematics of this particular fault segment can be determined from detailed topographic levellings. The fault strikes N30°E and dips 75° to the NW. Depending on their size and nature, transverse morphological features such as tributaries of the Rio Pangor and intervening ridges, reveal right-lateral offsets which cluster around 27 ± 11m, 41.5 ± 4 m, 590 ± 65 m and 960 ± 70 m. The slip vector deduced from the short-term offsets shows a slight reverse component with a pitch of about 11.5° SW. The 41.5 ± 4 m displacements are assumed to be coeval with the last glacial termination, yielding a mean Holocene slip-rate of 2.9- 4.6 mm yr−1. Assuming a uniform slip rate on the fault in the long term, the 27 m offset appears to correlate with an identified middle Holocene morphoclimatic event, and the long term offsets of 590 m and 960 m coincide with the glacial terminations at the beginning of the last two interglacial periods.

Stratigraphy and paleontology of upper Pleistocene deposits in the interandean depression, northern Ecuador

In the Interandean Depression of northern Ecuador (Carchi Province), more than 50 m of pyroclastic and windblown deposits (Cangahua Formation) were deposited during the cold phases of the Quaternary, interrupted many times by the development of evolved paleosoils during interstadials. The deposition occurred during downcutting if the plateau and covered existing morphologic irregularities. The upper part of the Cangahua Formation includes three fossiliferous horizons that contain fauna of Lujanian Mammal Age. On the basis of geomorphologic, sedimentologic, and paleontologic evidence, the upper part of the Cangahua is referred to the latest Pleistocene. After this, a renewed phase of downcutting began, reflecting early Holocene climatic amelioration.