Subandean Zone Research Articles

First record of a mosasaurid (Squamata: Mosasauridae) from the Upper Cretaceous of Ecuador

We report the first Upper Cretaceous marine reptile fossil in Ecuador, identified as an incomplete right maxilla from the Mosasauridae family. The remains were unearthed from marine marlstones and limestones within a karstic cave in the Tamia Yura indigenous community, situated northeast of Tena in the central Subandean Zone of the Ecuadorian Amazon. Morphological analysis reveals seven preserved teeth conical in shape. Stratigraphic analysis suggests the remains belong to the Middle Napo Fm, Oriente basin, indicating a presence from the Turonian to the end of the Coniacian. This discovery offers new insights into the study of Cretaceous marine reptiles in South America.

Ancestral cultures of the lower basin of the Santiago River, an approach from the Apinkras Creek, in Villa Gonzalo (district of Río Santiago, Condorcanqui-Amazonas)

In this research we dive into an area scarcely known by Amazonian and Peruvian archaeology, the Kanu sor Santiago River basin, the border area between the Sub-Andean Zone and the Amazonian plain. Through the recognition of an archaeological site in Apinkras Creek, current territory of the Wampis community of Villa Gonzalo (district of Río Santiago, Condorcanqui-Amazonas) and a subsequent analysis of the associated cultural material, we seek to approach the antiquity and the characterization of the societies that elaborated these objects during their permanence in the area. In this way, we identified that our study area was situated within a large area of interaction, mainly in the Amazon, at dates not yet precisely identified, but l estimated to have occurred between 2000 BC and 400 AD.

Neogene to modern foreland basin development in the Sub-Andean zone of southern Bolivia and northern Argentina, 21−23°S

The Sub-Andean retroarc region is a unique example of an active continental-scale retroarc foreland basin system. Heavily targeted for hydrocarbon exploration, the region hosts a large volume of subsurface data coupled to surface studies dedicated to refining its evolution in time and space. This paper presents a regional correlation of stratigraphic markers from seismic reflection and well logs across the Sub-Andean foothills at 23−21°S in southern Bolivia and northern Argentina, which reveals the contrasting along-strike history of Mesozoic to Cenozoic tectonics that preceded the foreland basin setting. Supported by published geochronological data and new zircon U-Pb maximum depositional ages, we describe the depositional transition from pre-Andean to Andean stratigraphy and discrete episodes of foreland basin subsidence and shortening. Based on interpreted stratigraphic breaks, we define the extent and stepwise evolution of this foreland basin, which was characterized by the progressive eastward migration of foreland basin depozones. Based on restored thickness profiles, we present flexural models of basin subsidence for the Sub-Andean foothills region. The modeling of discrete episodes of foreland basin subsidence refines the widely accepted bimodal elastic strength in the foreland basin at 21−23°S, which is weaker in the western ranges (∼20 km effective elastic thickness) and stronger eastward (>40 km). Modeling results also reveal minimum values of subsidence rates (up to 1.2 mm/yr) in the sequential foredeep depozones and suggest that the modeled tectonic load migration—as constrained by the record of syntectonic strata—probably increased over time through the incorporation of Sub-Andean rocks into the orogenic wedge.

New constraints on the Late Miocene-Pliocene deformational and depositional evolution of the Eastern Cordillera and Sub-Andean Zone in Southern Peru

The deformation history of the Eastern Cordillera (EC) and Sub-Andean Zone (SAZ) of southern Peru is critical for understanding the roles that tectonics and climate played in the erosional exhumation of bedrock and the associated sediment flux delivered to the Amazon drainage basin. In this study, we report new field and subsurface data, apatite fission track (AFT) and (U–Th)/He thermochronological ages, U–Pb ages on detrital zircon grains, Sr–Nd isotopic compositions of fine sediments, which combined with previously published data, provide new constraints on the Neogene deformation and deposition history across the southern Peruvian EC and SAZ between 12° and 14°S. Late Miocene-Pliocene deformation is recorded by AFT and AHe ages in both the EC and SAZ and by growth strata geometry in the SAZ. This period is characterized by the development of piggy-back synclines and duplexes in the SAZ, overthrusting of the EC, and input of recycled sedimentary rocks of the SAZ and first cycle sediments from Triassic Plutonic rocks from the EC to the late Miocene-Pleistocene piggy-back syncline sediments. We report a transition from low-energy sand-dominated to high-energy conglomerate-dominated deposits in the Plio-Pleistocene marking an increase in sedimentation rates, indicating that the thrust wedge has continued to propagate. Our data agrees with that of previously published studies and indicate that the Late Miocene-Pliocene was a period of tectonic uplift and deformation in both the EC and SAZ.

Assessing drivers of high exhumation magnitudes and young cooling ages in the eastern central Andes, southern Peru (13–18°S)

The potential structural controls on exhumation across the southern Peruvian Andes are not well understood, in part due to limited structural studies that co-locate with thermochronometric datasets. We integrate these two datasets and evaluate the relative contribution that fault geometry, magnitude, and shortening rate have on predicted cooling ages. Here we present a balanced cross-section constructed using new structural observations. This section, combined with existing thermochronometer data and a thermokinematic model, investigates the drivers of high exhumation and young canyon thermochronometric ages along the deeply incised Marcapata canyon in southern Peru. Together, these approaches constrain the timing and magnitude of exhumation in this portion of the southern Peruvian Andes and provide a mechanism for documenting how the internal architecture changes along strike.The balanced cross-section (oriented N30E) covers the Subandean Zone to the northeast, the Marcapata canyon on the eastern flank of the southern Peruvian Andes, and the Altiplano-Eastern Cordillera boundary to the southwest (13–18° S). Exhumation is constrained by four low-temperature thermochronometer systems, including apatite and zircon (U-Th)/He (AHe and ZHe, respectively) and fission-track (AFT and ZFT, respectively). The youngest AHe (∼1–3 Ma), AFT (∼3–7 Ma), ZHe (∼4–7 Ma), and ZFT (∼14–17 Ma) ages are located in the center and valley bottom of the Marcapata canyon. The thermokinematically modeled cross-section produces cooling ages determined by fault geometry and kinematics. Reset ZFT ages require burial of Ordovician rocks in excess of 5.5 km above the original 6.5 km depositional depth. We find that the ZFT and ZHe ages in the Eastern Cordillera are sensitive to the history and magnitude of burial, age and location of uplift, and canyon incision. Canyon incision is required to reproduce the youngest canyon thermochronometric ages while slow shortening rates from ∼10 Ma to Present are required to reproduce interfluve thermochronometric ages. Shortening is accommodated by basement faults that feed slip up through three different décollement levels before reaching the surface. The proposed stacked basement geometry sets the first-order cooling signal seen in modeled ages. We determined that the total shortening in this section from the Subandean Zone to the Altiplano is 147.5 km, similar to shortening estimates in an adjacent thermo-kinematically modeled section in the San Gabán canyon 50 km to the southeast. Both the ZHe and ZFT ages in the Marcapata section (4–5 and 14 Ma) are noticeably younger than cooling ages from the San Gabán section (16 and 29 Ma). The Marcapata section's higher magnitude of exhumation is due to a repetition of basement thrusts that continues to elevate the Eastern Cordillera while active deformation occurs in the Subandean Zone. The youngest thermochronometric ages in all four systems are co-located with the overlapping basement thrust geometry. This basement geometry, kinematic sequence of deformation, and canyon incision co-conspire to produce the young cooling ages observed in the Eastern Cordillera.

The Role of Isostasy in the Evolution and Architecture of Fold and Thrust Belts

Abstract Warmer conditions prevalent in the hinterland of orogenic systems facilitate local ductile flow underneath the surface load, making Airy-like local isostasy more prevalent in these domains. In contrast, flexural isostasy better describes the regional response to surface loading of more rigid lithospheres. Here, we explore how the interaction between horizontal tectonic mass transfer and vertical isostatic mass transfer, through either elastic flexure or viscous flow, impacts the overall architecture of fold and thrust belts. We compare numerical models of fold and thrust belts under either an Airy-like ductile isostasy boundary condition or a flexural-like regional isostasy boundary condition. Our experiments suggest that when ductile flow is involved in accommodating isostatic adjustment, subsidence is rather local, larger, and results in narrower, less elevated fold-thrust belts with a complex internal architecture consisting of prominent steeply dipping faults. When isostatic subsidence is controlled by lithospheric flexure, the tilting of the basement on 10 s of km scale facilitates the outward propagation of fold-thrust belts. The internal architecture is simpler and involves prominent basement-parallel décollements. The outcome is wider fold and thrust belts with higher topographies. A change in lithospheric elastic thickness does not significantly affect fold-thrust belt structural styles. Our results are compared to natural examples from the Subandean zone.

The Devonian Gamoneda section of southern Bolivia: new biostratigraphical and palaeobiogeographical data

Reconsideration of the Devonian marine lithological succession in the Gamoneda section of southernmost Bolivia evidences that the distinction of the Gamoneda Formation is not justified. Corresponding deposits have to be assigned to the Icla Formation of the central Subandean Zone. Indeed, in the Gamoneda area the local Devonian sequence only differs from that of the typical lcla and Presto areas in the better development of sandy levels. The occurrence of the Lower Devonian Scaphiocoelia boliviensis Zone in the lowermost beds of the section, just above the Santa Rosa Formation, is established for the first time. An eodevonariid chonetoidean brachiopod (Lomaella sp. cf. L. primoris) is recorded for the first time from Bolivia, and a new fissiculate blastoid is described: Brachyschisma? gamonedensis. The whole fauna allows to establish correlations both with the central Subandean Zone and with the Argentine Precordillera. Discovery of extra Malvinokaffric fauna) elements raises again the palaeobiogeographical relationships between these areas.

Stability Assessment of a Bedding Rock Slope Using Q-slope and Seismic Tomography: A Case Study in the Ecuadorian Amazon

Roads are generally affected by slope failures, and these failures can increase when there are weathered materials and high rainfall. These circumstances occur in the sub-Andean zone of Ecuador. This is the region where the study area is located. The stability of a stratified rock slope, which is affecting a section of highway E45, was evaluated. The study slope is exposed to the road, but the upper part is covered by a soil-type material and dense vegetation that makes it challenging to study. We applied the Q-slope method and seismic tomography; these methods used together worked well, because they allowed to correlate and infer information about the quality of the rock mass, even in a fast and economical way. We also performed core drilling with core recovery in the crown of the slope and SPT test. The slope presented two well-differentiated zones; therefore, Q-slope values were calculated for each of these zones. The results show that the slope is unstable. The application of seismic tomography as an input parameter for calculating Q-slope was important because it allowed evaluating the stability where it is impossible to collect geomechanical information, correlate information taken at the foot of the slope, and define the depth of the bedrock.

New observations on the recent eruptive activity of Sumaco Volcano (Ecuador), based on geochronology, stratigraphy and petrography

Sumaco is a stratovolcano located in the northern SubAndean zone of Ecuador, 105 km east of Quito. The flanks of Sumaco are jungle-covered, and scarce outcrops are found along stream banks, trails and, in quarries. Based on our field studies, eruptions of Sumaco during the historical era were associated with small-scale Strombolian activity. Earlier more robust activity, which pertains to the last 4400 years, had vigorous eruptions whose ∼10 cm thickness ash fallout deposits are identified more than 20 km from the vent.Stratigraphic sections studied around the volcano's outer flanks identify five principal ash-fall layers corresponding to Sumaco's activity. The ash layers contain white micro-vesiculated pumice, displaying clinopyroxene, plagioclase, traces of amphibole, and biotite crystals. Also, gray lithics with pyroxene phenocrysts and scarce plagioclase crystals are present. The amount of volcanic glass shards is meager in almost all samples. The thickest ash layer, “SUM-JS-44″, was dated by 14C, obtaining an age of 3029-2888cal AD.Analysis of a sediment core obtained from the “Guagua Sumaco” near-source lagoon allowed us to identify nine ash layers; seven are from Sumaco. Dating an underlying layer of peat (OL3) beneath the six ash layers provides an age of 1483–1644 cal AD, showing that Sumaco volcano experienced at least six separate eruptive pulses during the last centuries, and thus it should be considered potentially active.Based on morphological observations, Sumaco's edifice consists of three phases: PaleoSumaco, Recent Sumaco, and Current Sumaco, differentiated mainly by two scars representing flank collapses that produced far-reaching debris avalanches, whose breccia deposits are observed around the cone's depositional apron.The aim of our research is to understand the evolution of Sumaco's edifice and verify the relative youthfulness of the volcano and its eruption styles. These inputs are then applied in determining three eruption scenarios.

Jurassic to Early Paleogene sedimentation in the Amazon region of Ecuador: Implications for the paleogeographic evolution of northwestern South America

The retroarc Amazon region of Ecuador recorded distinct events that are preserved in the Mesozoic-Cenozoic sedimentary record of the Subandean Zone and Oriente Basin. In this contribution, we review the Jurassic to Paleocene geologic and paleogeographic evolution of the Ecuadorian foreland using stratigraphic, sedimentologic, provenance, and geochronological data. Our results provide new insights into the sedimentary systems, stratigraphic framework, depositional ages, and sediment dispersal patterns of retroarc basin fill. During the Early Jurassic, marine deposits of the Santiago Formation formed a carbonate platform in eastern Ecuador. At ~180 Ma, an eastward-prograding deltaic system transported volcaniclastic material from a western volcanic arc. Nonmarine conditions prevailed during most of the Middle to Late Jurassic, characterized by deposition of red beds and intercalated volcanic and volcaniclastic deposits of the Chapiza Formation. Clastic and volcanic deposition dated from ~160 to 130 Ma was coeval with regional extension, as suggested by sedimentologic and seismic data from the Oriente Basin. Provenance results show that a ~ 120 Ma fluvial system at the base of the Hollín Formation was initially sourced from the west and progressively evolved to a mixed source with contributions from the Amazonian Craton and incipient Andean topography. Deposition of the Hollín Formation occurred in a northwest-flowing fluvial system that evolved into an estuary system confined by the Andes to the west and the Amazonian Craton to the east. Coastal to shallow marine deposits of the Napo Formation accumulated during regional sea-level variations throughout the Late Cretaceous. The geometry of coastal deposits and supporting provenance data suggest a drainage system characterized by west-flowing rivers derived chiefly from the Amazonian Craton. The latest Cretaceous (~70 Ma) onset of shortening in the Northern Andes coincided with collision of the Caribbean Plateau. This tectonic episode induced a continental-scale drainage reorganization documented in provenance data showing an Andean origin of the east-directed fluvial deposits of the Maastrichtian–Paleocene Tena Formation.

Reconciling Spatial and Temporal Patterns of Cenozoic Shortening, Exhumation, and Subsidence in the Southern Bolivian Andes

The Bolivian Andes are an archetypal convergent margin orogen with a paired fold-thrust belt and foreland basin. Existing chronostratigraphic constraints highlight a discrepancy between unroofing of the Eastern Cordillera and Interandean Zone fold-thrust systems since 40 Ma and the onset of rapid sediment accumulation in the Subandean Chaco foreland after 11 Ma, previously attributed to Miocene climate shifts. New results from magnetostratigraphic, backstripping, erosional volumetric calculations, and flexural modeling efforts are integrated with existing structural and thermochronologic datasets to investigate the linkages between shortening, exhumation, and subsidence. Magnetostratigraphic and backstripping results determine tectonic subsidence in the Chaco foreland basin, which informs flexural models that evaluate topographic load and lithospheric parameters. These models show that Chaco foreland subsidence is consistent with a range of loading scenarios. Eroded volumes from the fold-thrust belt were sufficient to fill the Chaco foreland basin, further supporting the linkage between sediment source and sink. Erosional beveling of the Eastern Cordillera, local intermontane sediment accumulation after 30–25 Ma, and regional development of the high-elevation San Juan del Oro geomorphic surface from 25 to 10 Ma suggest that the western Eastern Cordillera did not store the large sediment volume expected from erosion of the fold-thrust belt, which arrived in the Subandean Zone after 11 Ma. Eocene to middle Miocene foreland basin accumulation was likely focused between the Eastern Cordillera and Interandean Zone, and has been almost completely recycled into the modern Subandean foreland basin. The delay between initial fold-thrust belt exhumation (early Cenozoic) and rapid Subandean subsidence (late Cenozoic) highlights the interplay between protracted shortening, underthrusting, and foreland basin recycling. Only with sufficient crustal shortening, accommodated by eastward advance of the fold-thrust belt and attendant underthrusting of Brazilian Shield lithosphere beneath the Subandes, did the Subandean zone enter proximal foreland basin deposystems after ca. 11 Ma. Prior to the late Miocene, the precursor flexural basin was situated westward and not wide enough to incorporate the distal Subandean Zone. These results highlight the interplay between a range of crustal and surface processes linked to tectonics and Miocene climate shifts on the evolution of the southern Bolivian Andes.

Crustal thickness and magma storage beneath the Ecuadorian arc

The Northern Andes of Ecuador contain some of the most active volcanic systems in the Andes and extend over a broad region from the Western Cordillera to the Subandean Zone. While it is known that the arc straddles a range of basement compositions, from accreted mafic oceanic terranes in the west to silicic continental terranes in the east, the details of the crustal structure beneath the arc is unclear despite being critical for understanding magmatic and tectonic processes in this portion of the Andes. To gain insight into these processes, we create two 3D models of crustal and upper mantle seismic properties throughout the region. The first highlights the discontinuity structure using receiver functions, which allows for the recovery of crustal thickness beneath the Ecuadorian Andes. We observe a range from ~50 to 65 km under the high elevations, with thicker crust beneath the lower elevation Western Cordillera compared to the higher elevation Eastern Cordillera. This can largely be explained by density variations within the crust that are consistent with observed terranes at the surface, implying these terranes extend to depth. The second model combines our receiver functions with Rayleigh wave dispersion data from ambient noise measurements in a joint inversion to construct a 3-D shear wave velocity model. This model shows several mid-crustal (5–20 km below sea-level) low velocity zones beneath Ecuadorian arc volcanoes that contain a maximum of ~14% melt. These low velocity zones likely represent zones of long-term magma storage in predominantly crystalline reservoirs, consistent with “mush zones”. Furthermore, the depth of the inferred reservoirs below several of the volcanic centers (e.g., Chiles-Cerro Negro and Tungurahua) are in broad agreement with previous geobarometry and geodetic modeling. Our results provide new observations of possible long-term magma reservoirs below other less-studied volcanic systems in the Ecuadorian arc as well, and further contributes to a mounting number of observations indicating long-term magma storage at low melt percentages in the mid-crust beneath active arc systems.

Kinematic deformation model of the south-central subandean zone, Bolivia

The Bolivian Subandean region is characterized as a thin-skinned fold-thrust belt with structures persisting for 50–100 km along strike in a broadly North-South direction. The evolution of the Subandean structures is related to shortening and mass transfer. The stratigraphy comprises ~12 km of Ordovician to Recent sediments, which are dominated by the Paleozoic section including Devonian source and reservoir horizons capped by the Middle Devonian Los Monos regional seal. Detachments occur at top of basement, dipping 2° to the west, within the Silurian and two Devonian levels, including the Los Monos shale.The fold-belt contains the Mega Gas Fields of Bolivia. This study focused on an exploration target in the Igüembe structure, located nearby the Margarita and Incahuasi Gas Fields. In addition to the standard methodology of integrating seismic lines, surface geology and well data, this study also utilized magnetotelluric (MT) analysis to create regional structural sections that were balanced and restored, recording ~40% shortening between 12Ma and the present. The compression was propagated from west to east along a basal detachment along top basement at ~12 km depth, creating four different and superimposed structural units. The Ordovician to Middle Devonian section deformed as antiformal stacks at ~15 km spacing, with the highly ductile Los Monos horizon overlying it and acting as a major detachment and seal. The Upper Devonian to Cretaceous section was buttressed by over 5 km of overlying Neogene sequence; in such a way, that it was forced to propagate to the surface directly above the antiformal imbricates in a sequence of triangle zones above and along the Los Monos detachment. The 5+ km thick Neogene section is preserved as parallel bedded open synclines between the triangle zone culminations. The Middle Devonian source and reservoir may have been buried 6–8 km prior to deformation, which has reduced porosity, but also generated hydrocarbons. It is plausible that during the Siluro-Devonian Cordilleran compressional phase, gentle regional anticlines or inversion structures might have been formed, perhaps with ~15 km spacing. These may have trapped the early-formed hydrocarbons and preserved porosity. These anticlines would be the nuclei for subsequent contractional deformation in the Neogene, with hydrocarbons re-migration and some hydrocarbons remnants trapped in the original structures.

Detrital zircon U-Pb geochronology of modern Andean rivers in Ecuador: Fingerprinting tectonic provinces and assessing downstream propagation of provenance signals

AbstractRecognizing detrital contributions from sediment source regions is fundamental to provenance studies of active and ancient orogenic settings. Detrital zircon U-Pb geochronology of unconsolidated sands from modern rivers that have source catchments with contrasting bedrock signatures provides insight into the fidelity of U-Pb age signatures in discriminating tectonic provenance and downstream propagation of environmental signals. We present 1705 new detrital zircon U-Pb ages for 15 samples of unconsolidated river sands from 12 modern rivers over a large spatial extent of Ecuador (∼1°N–5°S and ∼79°–77°W).Results show distinctive U-Pb age distributions with characteristic zircon age populations for various tectonic provinces along the Andean convergent margin, including the forearc, magmatic arc, and internal (hinterland) and external (foreland) segments of the fold-thrust belt. (1) Forearc and magmatic arc (Western Cordillera) river sands are characterized by Neogene–Quaternary age populations from magmatic sources. (2) Rivers in the hinterland (Eastern Cordillera) segment of the Andean fold-thrust belt have substantial populations of Proterozoic and Paleozoic ages, representing upper Paleozoic–Mesozoic sedimentary and metasedimentary rocks of ultimate cratonic origin. (3) River sands in the frontal fold-thrust belt (Subandean Zone to Oriente Basin) show distinctive bimodal Jurassic age populations, a secondary Triassic population, and subordinate Early Cretaceous ages representative of Mesozoic plutonic and metamorphic bedrock.Detrital zircon U-Pb results from a single regional watershed (Rio Pastaza) spanning the magmatic arc to foreland basin show drastic downstream variations, including the downstream loss of magmatic arc and hinterland signatures and abrupt introduction and dominance of selected sources within the fold-thrust belt. Disproportionate contributions from Mesozoic crystalline metamorphic rocks, which form high-elevation, high-relief areas subject to focused precipitation and active tectonic deformation, are likely the product of focused erosion and high volumes of local sediment input from the frontal fold-thrust belt, leading to dilution of upstream signatures from the hinterland and magmatic arc.

Depositional environments and landscapes of the upper Miocene Ipururo Formation at Shumanza, Subandean Zone, northern Peru

During the late Miocene, the Andean–Amazonian region experienced drastic climatic and environmental changes, notably due to a major phase in the Andean uplift. The fossil record is virtually undocumented for this period in the Subandean Zone, where very few palaeoenvironmental and palaeontological investigations have been undertaken. Here, we describe plant remains (pollen, spores, and leaves), microfossils, mollusks, and vertebrates from the Ipururo Formation at Shumanza, San Martín, Peru. Twenty-nine plant families are identified from 164 pollen grains and 89 spores, among them Lycophytes, Monilophytes, and angiosperms (5 monocots and 18 eudicots). The pollen sample notably includes Grimsdalea magnaclavata, Palaeosantalaceaepites cingulatus, Echitricolporites spinosus, and Fenestrites longispinosus, pointing to a late Miocene–early Pliocene age for the TAR-27 locality (10.06–3.72 Ma). Leaf impressions, from nearby localities in the same section, document Malvaciphyllum sp. (Malvaceae), three morphs resembling Caryocaraceae, Fabaceae, Myrtaceae, and two unidentified ‘Dicotyledonae’ angiosperms. The mollusk assemblage is somewhat reminiscent of early–middle Miocene Pebasian faunas and dominated by gastropods (ampullariids, cochliopid, cerithioid, and planorbids). It also includes sphaeriid and unionoid bivalves. Vertebrate recovery is very poor, with a serrasalmine characiform and unidentified actinopterygian teeth. Fossil assemblages and sedimentary facies consistently testify to the dominance of riverine/alluvial forests and the persistence of a steady lowland rainforest close to the Andes less than 10.1 million years ago, without indication of (1) mangrove/marine environments or (2) high-elevation ranges in the close surroundings of Shumanza by that time. By coupling palynostratigraphy and lithostratigraphy, Shumanza fossil assemblages would be further assigned an early late Miocene age (10.1–ca. 8 Ma).

3D model related to the publication: A platyrrhine talus from the early Miocene of Peru (Amazonian Madre de Dios Sub-Andean Zone)

This contribution contains the 3D model of the fossil talus of a small-bodied anthropoid primate (Platyrrhini, Cebidae, Cebinae) discovered from lower Miocene deposits of Peruvian Amazonia (MD-61 locality, Upper Madre de Dios Basin). This fossil was described and figured in the following publication: Marivaux et al. (2012), A platyrrhine talus from the early Miocene of Peru (Amazonian Madre de Dios Sub-Andean Zone). Journal of Human Evolution.

The “Mera” lahar deposit in the upper Amazon basin: Transformation of a late Pleistocene collapse at Huisla volcano, central Ecuador

The Sub-Andean zone east of the Cordillera Real, Ecuador and out to the Amazon basin's western margin has been the depository of voluminous lahars related to volcanic activity in the Andean highlands. These lahars arrived to the Sub-Andean zone via gravitational transport through narrow river canyons and emplaced volumes surpassing several cubic kilometers over robust inundation zones. This paper discusses the origin, flow route, depositional zone, terrace formation and geomorphic significance of the most important lahar deposit yet mapped in this region, that of the Mera lahar, which likely formed from a late Pleistocene collapse of Huisla volcano, followed by impounding by temporary dams behind drainage-blocking debris avalanche deposits (DAD), then subsequent rupture of the blockage. The actual deposit of the Mera lahar has a thickness between 30 and 70 m, is mainly comprised of DAD breccia, is matrix-supported (c 70%), of reddish gray color and is well-consolidated. Based on geochemical and petrographic similarities, Huisla volcano's DAD is the best candidate for the lahar's source. Huisla volcano is located some 90 km up valley of the bulk of Mera lahar's mapped deposit. Clasts of Mera lahar rocks and Huisla's DAD breccias have 57–61 wt% SiO2, corresponding to andesites of the calc-alkaline series with mean values of 1–1.5 wt% for K2O. The mapped Mera lahar deposit has an actual volume estimated in 1.2 km3 compared to its original estimated volume of 5.4 km3. Cross-sectional widths of 1.5 to 4.5 km span and extend laterally beyond the actual Pastaza river channel where the lahar's deposition produced high-standing isolated surfaces that are notable local geomorphic features of the upper Sub-Andean zone. The flow modeling program LaharFlow, employing the modern landscape as the topographic base, adequately simulates the flow route and inundation zones of the Mera lahar.

Shortening and structural architecture of the Andean fold-thrust belt of southern Bolivia (21°S): Implications for kinematic development and crustal thickening of the central Andes

Reliable crustal shortening estimates for the central Andes (South America) are a critical component in validating models of Cordilleran processes. In southern Bolivia, insight into crustal shortening and the kinematic development of the Andean thrust belt are limited by the lack of a unified structural evaluation across the entire width of the retroarc region. To address these shortcomings, we (1) estimate crustal shortening by integrating new geologic mapping with published geophysical data to construct a balanced cross section across the Subandean zone (SAZ), Interandean zone (IAZ), and Eastern Cordillera (EC) at 21°S; (2) develop a kinematic model for the retroarc thrust belt; and (3) estimate crustal budgets and average crustal thicknesses over the region. We estimate 337 ± 69 km (36% ± 7%) of total shortening (SAZ, 82 km; IAZ, 70 km; EC, 120 km; Altiplano, 65 km). The thrust belt developed from late Eocene time to the present by tectonic wedging and eastward emplacement of two ∼10–12-km-thick basement thrust sheets that distribute slip into overlying sedimentary rocks. Our range of crustal shortening values can account for 90%–118% of the current retroarc crustal area. Assuming an initial crustal thickness of 35 km, the EC and Altiplano did not achieve modern crustal thicknesses (∼65 km) until the present. However, assuming a 40-km-thick initial crust, the EC and Altiplano attained the critical thickness for either eclogitic phase changes or lower crustal flow (>45–50 km) by ca. 27–25 Ma, modern thicknesses by ca. 10 Ma, and currently exceed geophysically observed thicknesses by ∼2.5–14.5 km; this suggests crustal losses significant enough to have affected hinterland surface elevation.

Thrust tectonics in the Andean retro-foreland basin of northern Peru: Permian inheritances and petroleum implications

In northern Peru, the Huallaga-Moyabamba-Marañon Subandean foreland basin system results from the interaction between thin and thick-skinned tectonics. Geophysical data and the construction of two balanced cross-sections show that this structural configuration has been controlled by Permian inheritances. A fossilized west-verging Middle Permian fold and thrust belt, which developed during the Gondwanide orogeny, has been partly reactivated by the Andean compression and controlled thick-skinned tectonics propagation. This west-verging thrust system is still active and causes the crustal and damaging seismicity of the Moyabamba region. Late Permian salt, which has sealed the Middle Permian fold and thrust belt, controlled thin-skinned tectonics propagation and the development of the must large overthrust of the Peruvian Subandean zone. The fossilized and partly reactivated Middle Permian fold and thrust belt constitutes a new petroleum play for the exploration in the northern Peruvian Subandean basins. Sub-thrust traps of the Moyabamba and Huallaga wedge-top basins are particularly attractive but stay unexplored.

Crustal deformation and surface kinematics after the 2010 earthquakes in Latin America

Strong earthquakes cause large changes in the station positions and velocities of the geodetic reference stations; i.e., the global ITRF (International Terrestrial Reference Frame) and its regional densifications like SIRGAS (Sistema de Referencia Geocéntrico para Las Américas) in Latin America and the Caribbean. To ensure the long-term stability of the geodetic reference frames, the transformation of station positions between different epochs requires the computation of reliable continuous surface deformation (or velocity) models. This paper presents the computation of a new continental continuous crustal deformation model for Latin America and the Caribbean inferred from GNSS (GPS+GLONASS) measurements gained after the strong earthquakes occurred in 2010 in Chile and Mexico. It is based on a multi-year velocity solution for a network of 456 continuously operating GNSS stations and covering a five years period from March 14, 2010 to April 11, 2015. This new deformation model, called VEMOS2015 (Velocity Model for SIRGAS 2015), is computed using the least square collocation (LSC) approach with empirically determined covariance functions. The result is summarised as follows: While the effects of the Baja California earthquake can be considered as local, the effects of the Maule earthquake changed the surface kinematics of a large area (between the latitudes 30°S–45°S from the Pacific to the Atlantic coasts). Before the Maule earthquake, the strain rate field in this area showed a strong west-east compression with maximum rates of about 0.40μstrain/a between latitudes 38°S and 44°S. In accordance, the deformation vectors were roughly parallel to the plate subduction direction and their magnitudes decreased with the distance from the subduction front. After the earthquake, the largest compression (0.25μstrain/a) occurs between the latitudes 37°S and 40°S with a N30°E direction. The maximum extensional strain rate (0.20–0.35μstrain/a) is observed in the Sub-Andean zone in the Patagonia south of latitude 40°S. The extensional axes rotate from a N30°E direction in the central Araucania zone to a westerly direction of N72°W in the western part of Patagonia. In the northern region of parallel 35°S, the extension is also directed to the Maule zone (S45°W) but with quite smaller rates (<0.06μstrain/a). This complex kinematics causes a large counter clockwise deformation pattern rotating around a point south of the epicentre (35.9°S, 72.7°W). The magnitude of the deformation vectors varies from 1mm/a close to the rotation point up to 22mm/a near the 2010 earthquake epicentre. The direction of the largest deformation vectors points to the epicentre. VEMOS2015 covers the region from 55°S, 110°W to 32°N, 35°W with a spatial resolution of 1°×1°. The average prediction uncertainty is ±0.6mm/a in the north-south direction and ±1.2mm/a in the east-west direction. The maximum is ±9mm/a in the Maule deformation zone while the minimum values of about ±0.1mm/a occur in the stable eastern part of the South American plate.