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Fluvial terraces as quantitative markers of late Quaternary detachment folding and creeping thrust faulting in the Peruvian Huallaga basin
(Elsevier, 2020-10-15)
In the western Huallaga basin, located in the Peruvian Subandes, a series of nine fluvial terraces with elevations of up to 160 m above current floodplain level (+FP) were formed against the Biabo anticline. The terraces are progressively more folded with age and terrace elevation and present growth strata, suggesting ongoing folding of the anticline. Optically and InfraRed Stimulated Luminescence (OSL and IRSL) dating of fluvial terrace sediments yielded ages of 10.8 ± 0.7 ka for the T2, 24.2 ± 2 ka for the T4 and 46 ± 9 ka for the T7 terrace. Uniform uplift rates of 2 mm a−1 for the past 46 ka were calculated. Terrace profiles and two new seismic profiles that cross the Biabo anticline longitudinally and transversally were used to calculate shortening rates of 2.6 mm a−1 for the anticline over the past 46 ka. Our calculations took into account potential contributions of detachment thrusting parallel to the Biabo fold and reverse tear faulting perpendicular to the fold tip as visible on the seismic sections. A maximum uplift rate of 0.62 mm a−1 due to reverse thrusting of the Bellavista tear fault for the past 46 ka could have contributed to the overall uplift of the Bellavista terraces. Based on trigonometric relationships, a reverse fault slip rate of 0.86 mm a−1 and an age of 137 ka were calculated for this fault. In the eastern part of the Huallaga basin, a series of two fluvial terraces were formed with elevations of 13–15 m and 30 m + FP and with OSL ages of 6.9 ± 0.7 ka for T1 and 30.2 ± 3.1 ka for T2. The interpretation of a third new seismic profile crossing the study area in combination with field observations and trigonometric relationships shows that the terraces were formed by a complex system of oblique ramp and strike-slip faulting resulting in uplift and shortening rates of 1 mm a−1 and 1.96 mm a−1 between 30.2 ka and 6.9 ka respectively; and uplift and shortening rates of 2 mm a−1 and 3.70 mm a−1 from 6.9 ka until the present. Reverse slip of the Shapaja oblique ramp, that was responsible for uplift of the terraces, was calculated at 2.19 mm a−1 for the period 30.2 to 6.9 ka and 4.15 mm a−1 from 6.9 ka until the present. Measured total slip in combination with the calculated slip rates allowed an age calculation of 181 ka for the Shapaja thrust ramp. Our inferences showed that the studied tectonic structures absorb most of the present-day deformation in the Huallaga basin. The calculated uplift and shortening rates are compatible with published, present-day crustal movement velocities, and Miocene to present shortening rates for various, studied sites of the tropical Subandes and suggest that it is deforming at a relatively uniform rate. The timing of sedimentation of the terrace sediments may have corresponded to periods of increased precipitation which occurred coeval with the Heinrich events of the Northern Hemisphere. Terrace records similar as ours have been described for other, tectonically highly active areas in the tropical Subandes, but not in the tectonically less active Amazonian lowlands. It is suggested that uplift rates of at least 1 mm a−1 are needed to preserve suborbital climate cycles as individual terraces. The tropical Subandes could therefore be a new key area to investigate the interactions between tectonics, suborbital climate forcing and fluvial response during the late Quaternary.
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Assessing the accuracy of combined DEM-based lineament mapping and the normalised SL-index as a tool for active fault mapping
(Elsevier, 2021-08-20)
Satellite image-based lineament mapping in combination with the normalised Stream Length Gradient Index (SL/K-index) is one of the most widely used methods to identify active faults in areas where other verification methods, such as faulted strata, geophysical subsurface data, or seismic activity in the upper crust, are unavailable, and to classify their degree of activity. The accuracy of this approach, however, has not yet been assessed rigorously. We assess its accuracy by comparing the results against 27 thrust fault segments that were identified from six seismic reflection profiles with a total length of 340 km. Over 106,000 SL/K points were calculated from 815 river profiles on basis of the SRTM v.3.0 DEM. Standard deviations (SD) of < -2SD (class 1) to > +2SD (class 5) were calculated from the lognormalised SL/K data and five SL/K classes were made and assigned to the nearby lineaments. A comparison with the fault segments from the seismic profiles showed a good coincidence between the presence of knickpoints, SL/K classes 4 and 5 and to a lesser extent class 3. Lineaments with SL/K classes 1 and 2 never corresponded to actual faults. We identified seven master thrusts of which three are very active to highly active, likely older, structures with important amounts of accumulated slip. The other thrusts are probably younger faults with relatively little accumulated slip, yet they are also active to highly active faults. Minor strike-slip faults were identified as very active to highly active structures. Published slip rates shows that at least some of the SL/K class 4 faults experience slip rates of ~1 to ~4 mm a−1 and the SL/K class 5 strike-slip faults may experience slip rates as high as one metre over the course of several months. Most active faults occur where the basal evaporite layer is abnormally thick.
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An assessment of competing factors for fluvial incision: an example of the late Quaternary exorheic Moyobamba basin, Peruvian Subandes
(Elsevier, 2021-05-01)
Three competing factors for late Quaternary fluvial incision in the Moyobamba piggyback basin, located in the northeastern Peruvian Subandes, were investigated, namely base-level change as a result of basin opening, changes in the ratio sediment load:discharge (Qs: Qw) controlled by orbital and suborbital climate cyclicity, and thrust-related tectonic uplift. To assess the relative importance of these factors, the fluvial terrace staircases of five rivers were studied in terms of their sedimentology, stratigraphy and geomorphology. A new dataset of eight Optically Stimulated Luminescence (OSL) and four 14C ages shows that the final phase of endorheic alluvial fan aggradation lasted until 18.0 ± 1.4 ka, after which basin-wide incision occurred. Incision at the basin outlet as a result of adaptation to the new base level is decoupled from the observed incision in the middle and upper parts of the basin. There, incision is the result of a northwestward increase in uplift related to movements of the Cerro Blanco thrust at the western border of the Moyobamba basin. Incision was continuous from 18 ± 1.4 ka until 10.3 ± 0.7 ka without terrace formation, but between 10.3 ± 0.7 ka and 5.5 ± 0.4 ka up to four terrace levels were formed. Deposition of fluvial terrace sediments may have occurred during short periods characterised by a more intense South American Summer Monsoon (SASM) controlled by suborbital climate cyclicity, but a more precise age control is needed to confirm this theory. Precession-controlled, increased Holocene aridification between 10.3 ± 0.7 ka and 5.5 ± 0.4 ka may have resulted in a decrease in the ratio Qs: Qw leading to a five-fold increase in fluvial incision rates of up to 6.0 mm a−1 and flattening of terrace profile gradients with rates of up to 2.6 mm a−1. After 5.5 ± 0.4 ka, rates dropped again. Over longer timescales, movement of the Cerro Blanco thrust caused basin-wide uplift and fluvial incision at a more constant pace. During the past ~20 ka, incision increases from the southwest basin border towards the northwestern basin border with corresponding uplift rates increasing from 0.2−0.2+0.3 mm a−1 to 2.1−0.2+0.2 mm a−1; and with shortening rates increasing northwestward from 0.2−0.2+0.4 mm a−1 to 2.8−0.2+0.3 mm a−1. The highest, presented uplift rates are in agreement with independently published data for our study area. Our data shows that over longer timescales of ~20 ka, incision rates can be used as proxies for tectonic uplift, but over shorter timescales incision rates may be seriously affected by climate change and cannot be used to infer uplift rates. It is further suggested that continuous uplift and erosion of the Cerro Blanco thrust system, and subsequent sedimentation in the basin, in combination with increased precipitation during Heinrich events 1 and 2 and the Last Glacial Maximum (LGM), led to basin overflow. Simultaneously, river capture through headward erosion, guided by strike-slip faulting, may have led to the definitive opening up of the Moyobamba basin. To our knowledge, this is the first time that late Quaternary basin opening has been demonstrated for the South American Andes and one of the few, known cases worldwide.
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Heinrich events and tectonic uplift as possible drivers for late Quaternary fluvial dynamics in the western Peruvian Andes
(Elsevier B.V., 2022-11-01)
Late Quaternary fluvial valley development in Peru is considered to be driven by summer insolation maxima in the precession cycle, which coincide with increased precipitation and rising lake levels in the central Andes. Our literature review, however, indicates that fluvial aggradation does not always coincide with summer insolation maxima but may also occur during transitions between insolation maxima and minima or even during summer insolation minima. Tectonic uplift as a driver of fluvial incision has not been considered in the current terrace formation models, despite the Peruvian margin being located above an active subduction zone. We present new chronologic, stratigraphic and geomorphic data of the Cañete River valley over the past 102 ± 6 ka. We mapped its fluvial terraces (11.5°-13° southern latitude) along a 75-km long reach perpendicular to the strike of the Andes. Five fluvial terraces and one floodplain level were identified with relative elevations of up to 181.1 m above the floodplain. Thirty-three (pIR) IRSL ages of eight fluvial terraces and one alluvial fan sediment sample showed that their ages in part correspond to summer insolation maxima of the precession cycle, but that a better match exists with pluvial periods that coincided with Heinrich events of the northern hemisphere and rising lake levels in the Andes. The chronology of terraces of the Cañete River agrees with those of previously studied fluvial systems between 7° and 16° southern latitude in Peru, suggesting a regional-scale fluvial response possibly to the Heinrich events. Reconstructed longitudinal profiles and terrace ages were utilised to calculate vertical incision rates. We calculated a mean rate of 1.8 ± 0.10 mm a−1 over the last 102 ± 6 ka, but incision rates varied considerably within this time period possibly in response to changes in the Qs: Qw ratio as a consequence of increased sediment input during the wet Heinrich events. A set of 1338 GNSS measurements of daily, vertical crustal deformation of the years 2009–2015 showed a continuous and positive trend in interseismic, accumulative vertical crustal movements with a time-averaged, crustal uplift rate of 1.9 ± 3.6 mm a−1. The positive trend in vertical movements, together with data from literature suggest landscape rejuvenation along the forearc and western Cordillera in central Peru. It is proposed that maximally ∼0.5 mm a−1 of the total incision over the past 102 ka may possibly be related to nonrecoverable, interseismic deformation. Climate change-driven variations in the Qs:Qw ratio are superimposed on the long-term tectonic uplift trend, and both are considered the main drivers of fluvial incision.
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Spatial variation in specific sediment yield along the Peruvian western Andes
(Elsevier BV, 2022-10-22)
The tropical Andes has been less studied in terms of erosion processes in comparison to other major mountain ranges in the world. Environmental gradients are steepest along the western flank of the Andes that is characterized by marked differences in vegetation, rough topography with deeply incised canyons, and highly variable and extreme precipitation patterns. Previous efforts mostly focused on sediment fluxes in large rivers draining to e.g. the Amazon basin while small to medium-sized rivers such as the ones flowing towards the Pacific Ocean have been relegated. They highlighted the link between sediment yield, anthropogenic and natural factors, e.g. climate, topography, river runoff, lithology and vegetation cover. In this study, we identified the spatial patterns of specific sediment yield along the western slopes of the Peruvian Andes between 3° and 13° S latitude for 21 catchments. We collected and analysed data from 22 environmental factors to elucidate their importance on spatially varying sediment yield. The sediment load was derived from gauging stations, reservoir sedimentation and water turbidity over a 30-yr period. The specific sediment yield varies strongly along the Peruvian western Andes as a consequence of the spatial variation in climate, topography and land cover controlling sediment production and transport. We reported higher-than-average specific sediment yields for the central part (6°-11°S) with values of 2130 and 2300 t km−2 yr−1 and low and uniform yields of 39 to 551 t km−2 yr−1 in the southern part (11° − 14.5° S). Given the scarcity of data on sediment yield, we included an uncertainty assessment based on bootstrapping approaches as to get a better grasp on the potential range of specific sediment yields in the study region. Using statistical techniques including Spearman correlation rank, univariate and multivariate regression analyses, we were able to determine the importance of the 22 environmental variables on the specific sediment yield. About 55 % of the observed variance can be explained by river discharge (Q90) and river steepness index (ks50). By adding an anthropogenic variable based on land cover, the explained variance in SSY increases up to 63 %, however, the effects of land cover on specific sediment yield are not clear because of spurious correlation between land cover, river discharge and topography. Our study therefore provides important new insights in the ongoing scientific debate on sediment yield variability in the western Andes.
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A 121-ka record of Western Andean fluvial response to suborbital climate cycles recorded by rhythmic grain size variations of the Lima fluvial fan
(John Wiley and Sons Ltd, 2024-06-30)
A complete, fluvial stratigraphic record for the last glacial period of the Western Andes in Peru is not available due to preservation issues and spatial variability in sedimentation. Deposits are typically restricted to incomplete records of fluvial terraces or localised occurrences of alluvial fans and landslides. These landforms are thought to have formed under a regime of climate cyclicity controlling increases in precipitation. Because of the fragmented preservation of these deposits, as well as dating uncertainties, it remains unclear if orbital climate cycles, such as the precession cycle, or suborbital cycles, such as the wet Heinrich events, are driving Andean sedimentation. In this paper, we try to answer this question through a sedimentological–stratigraphical analysis of a much more complete sedimentary sequence than usually found in the region. We present the results of a grain size analysis of 5000 clasts and 13 new luminescence ages of a 52-m-long, stratigraphic section of the Lima fluvial fan in Peru. Bayesian age–depth modelling resulted in a robust chronostratigraphic framework and derived sedimentation rates. The stratigraphic record registered sedimentation from 121.7 ± 4 to (Formula presented.) ka. Three major sedimentation periods occurred between 121.7 to (Formula presented.), 87 ± 1 to (Formula presented.), and (Formula presented.) to (Formula presented.) ka. These periods registered various unconformities and coarsening–fining upward sequences which chronologically correlate to suborbital pluvial periods, recognised from speleothems and lake records, that drove fluvial deposition. They also correlate with the timing of other recognised sedimentation events throughout the Western Andes. Marine regression resulted in fan progradation and not in incision. The Lima fan stratigraphy represents therefore the most complete, last glacial fluvial record for the Peruvian Western Andes to date and it highlights the potential of fluvial fans as recorders of suborbital climate variability.
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Quantifying Geomorphic Change in Andean River Valleys Using UAV-PPK-SfM Techniques: An Example from the Western Peruvian Andes
(Elsevier B.V., 2023-08-15)
The western Peruvian region is prone to erosion and geomorphic change. Extreme precipitation events lead to rapid change in river channel and floodplain morphology due to bank erosion and debris flows delivering detrital material to the fluvial system. Monitoring geomorphic events and their associated topographic changes at high spatial and temporal resolutions remains a challenge. Here, we used an Uncrewed Aerial Vehicle - Post-Processing Kinematic - Structure from Motion (UAV-PPK-SfM) approach that includes co-registration of point clouds by using relative Ground Control Points (GCPs). This workflow adjusts each elevation model to a reference model using invariant features that did not change their position or form over time. We applied this technique to monitor landscape change (2019–2021) in an area of 0.3 km2 located in the Cañete River basin. Our results showed that a minimum observable elevation change of 0.56 m (95 % confidence interval) can be achieved using this workflow, beyond which an actual elevation change can be separated from systematic error. Using object-based classification techniques on the aerial images, we separated geomorphic dynamics from land cover changes. This allowed us to isolate the effect of geomorphic processes, and quantify rates related to gully erosion, river scouring, bank erosion, and sediment deposition. Within the study area, a hotspot of geomorphic change corresponded to an ephemeral tributary channel. The gully channel incising an alluvial fan is highly dynamic, showing bank erosion of 0.75 to 3.2 m and net export of 37 m3 of sediment in the 25-month study period. Given that the monitoring period did not include high intensity rainfall events, the study illustrates how geomorphic activity in ungauged Andean river basins, such as the Cañete valley, may be considerably underestimated in literature.
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A Performance Comparison of SRTM v. 3.0, AW3D30, ASTER GDEM3, Copernicus and TanDEM-X for Tectonogeomorphic Analysis in the South American Andes
(Elsevier B.V., 2023-07-01)
Digital Elevation Models (DEMs) are widely used to assess the degree of tectonic activity in mountainous landscapes. But hardly ever have quality assessments of DEMs been carried out to assess their suitability for the calculation of the most widely used tectonogeomorphic indices. For that reason, we have analysed the five most commonly used DEMs for two tectonic basins in the Peruvian Andes. Those are the 30-m SRTM v.3.0, AW3D30, ASTER GDEM3, Copernicus and the 12-m TanDEM-X. The analysed indices are related to the characteristics of 22 drainage networks and we included a vertical accuracy assessment based on available GNSS control points. Copernicus produced the smoothest river profiles followed by AW3D30 and TanDEM-X. River profiles from the rainforest-covered Moyobamba tectonic basin were noisier than those from the more arid Huancayo tectonic basin. All DEMs performed statistically similar in the calculation of drainage basin area, θ, m/n, Ksn and the Hypsometric Integral. Copernicus and TanDEM-X generated the longest drainage networks. TanDEM-X showed the highest vertical accuracy with a RMSE of 3.174 m in the rugged Huancayo basin, and 2.172 in the Moyobamba basin, followed by AW3D30. Copernicus showed very uneven results between both tectonic basins. TanDEM-X allowed the most detailed mapping of fluvial and tectonic landforms, with the identification of six out of seven fluvial terraces, while Copernicus performed best of all 30-m DEMs. The overall best performing DEMs were Copernicus and TanDEM-X, closely followed by AW3D30. ASTER GDEM3 generally performed worst. In general, there was more statistical variability between DEMs in the more rugged Huancayo basin, suggesting that steeper slopes had a significant impact on the calculated indices. Our results provide a clear guideline for the scientific community of which DEMs to pick for the calculation of the various tectonogeomorphic indices.
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AI-based geological subsurface reconstruction using sparse convolutional autoencoders
(Elsevier BV, 2025-10-01)
Subsurface reconstruction is critical for geological modeling and resource exploration. Conventional spatial interpolation methods are limited by stationarity and spatial isotropy assumptions, while advanced geostatistical techniques require specialized datasets. Deep learning approaches often need large datasets, which is impractical for geoscientific applications. This study presents an AI-based methodology using a sparse convolutional autoencoder for robust subsurface modeling under data constraints and integrating secondary data sources such as Vertical Electrical Sounding (VES) data. A four-stage testing framework was implemented: (1) emulating conventional interpolation for baseline performance; (2) reconstructing subsurface geometries from synthetic data; (3) incorporating geophysical constraints through VES forward modeling; and (4) validating the methodology using a real-world case study from the Huancayo tectonic basin in the Peruvian Andes, using 41 VES measurements across two cross-sections (12 and 14 km long). Results demonstrate that the proposed model effectively emulates kriging interpolation (mean squared error: 1.5 × 10−3 to 1.2 × 10−3 with 100–800 training examples) through transfer learning from an inverse-distance, pre-trained model. In subsurface reconstruction, the model outperforms kriging (37.4–61.7 % improvement across 1–15 % sampling densities) through its ability to adapt to non-stationary conditions. When incorporating synthetic VES data, the model effectively reconstructed subsurface geometries with error reduction from 4.1 × 10−1 to 9.1 × 10−3 as stations increased from 1 to 40, demonstrating diminishing returns beyond this point. Application to the Huancayo basin case study validated the model's practical applicability by successfully identifying previously unmapped features including the contact between basement and sedimentary infill, folds and faults. The methodology demonstrates the AI's capability to enhance geological understanding in complex tectonic settings, revealing subtle features and refining existing assumptions about subsurface architecture.
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