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Canyon incision chronology based on ignimbrite stratigraphy and cut-and-fill sediment sequences in SW Peru documents intermittent uplift of the western Central Andes

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Abstract

Based on an ⁴⁰Ar/³⁹Ar- and U/Pb-based chronostratigraphy of ignimbrite sheets and the geomorphological features of watersheds, river profiles and slope deposits in the Ocoña–Cotahuasi–Marán (OCM) and Colca valleys of southwest Peru, we reconstruct the valley incision history of the western Central Andes over the last c. 25 Myr. We further document the Pleistocene and Holocene evolution of deep valleys on the basis of 14 ¹⁰Be surface-exposure ages obtained on debris-avalanche deposits and river straths. The data suggest that uplift was gradual over the past 25 Myr, but accelerated after c. 9 Ma. Valley incision started around 11–9 Ma and accelerated between 5 and 4 Ma. Incision was followed by several pulses of valley cut-and-fill after 2.3 Ma. Evidence presented suggest that the post-5 Ma sequence of accelerated canyon incision probably resulted from a combination of drainage piracy from the Cordilleran drainage divide towards the Altiplano, accentuated flexural tilting of the Western Cordillera towards the SE, and increased rainfall on the Altiplano after late Miocene uplift of the Eastern Cordillera. The valley deepening and slope steepening driven by tectonic uplift gave rise to large occurrences of rockslope failure. The collapsed rock masses periodically obstructed the canyons, thus causing abrupt changes in local base levels and interfering with the steadiness of fluvial incision. As a result, channel aggradation has prevailed in the lower-gradient, U-shaped Pacific-rim canyons, whereas re-incision through landslide deposits has occurred more rapidly across the steeper V-shaped, upper valleys. Existing canyon knickpoints are currently arrested at the boundary between the plutonic bedrock and widespread outcrops of middle Miocene ignimbritic caprock, where groundwater sapping favouring rock collapse may be the dominant process driving headward erosion.

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... First, it is particularly active geodynamically, related to the longterm convergence between the Nazca and the South America plates (Armijo et al., 2015;Martinod et al., 2020). This global shortening generates relief construction (Martinod et al., 2020) and produces instantaneous deformation (subduction and crustal seismicity; Villegas-Lanza et al., 2016) coupled with long-term processes of surface uplift (Thouret et al., 2017) and volcanism (Mamani et al., 2009). Second, the climate of this region is specific, with some places being one the driest worldwide: referred to as the Atacama Desert in Northern Chile and Southern Peru (Hartley and Chong, 2002). ...
... We bring here below further detailed geological and geomorphological descriptions of this key region required to better understand the gravitational destabilization affecting the Central Western Andes. Thouret et al., 2017;Aricota landslide -Delgado et al., 2020;Caquilluco landslide -Zerathe et al., 2017;Lluta landslide -Wörner et al., 2002;Strasser and Schlunegger, 2005;Miñimiñi and Latagualla landslides -Pinto et al., 2008;El Magnifico landslide -Mather et al., 2014;Crosta et al., 2017), (c) and (d) AA ′ topographic profile of the western flank of the Central Andes and its geological interpretation (adapted from Armijo et al., 2015), respectively. CC: Coastal Cordillera, CD: Central Depression, WC: Western Cordillera, AL: Altiplano. ...
... Indeed, the reviews of previous studies included individual landslide case studies (e.g. Strasser and Schlunegger, 2005;Pinto et al., 2008;Hermanns et al., 2012;Zavala et al., 2013;Margirier et al., 2015;Zerathe et al., 2016Zerathe et al., , 2017Bontemps et al., 2018;Lacroix et al., 2015Lacroix et al., , 2019Lacroix et al., , 2020Thouret et al., 2017;Delgado et al., 2020;Gaidzik et al., 2020;Sánchez-Núñez et al., 2020). We also revised a few studies including some local landslide databases such as (Audin and Bechir, 2006for South Peru and Mather et al., 2014. ...
Article
The western flank of the Central Andes hosts some of the largest terrestrial landslides (v > km³), which morphologies are particularly well-preserved due to low erosion rates related to the hyper-arid climate prevailing in this region since the Miocene. First-order questions are pending about the factors controlling the development and the triggering of those large-scale slope failures. Previous studies provided some geomorphological analysis and dating on individual study cases, but a regional-scale vision of landslide processes long the Central Western Andes is missing. Here we report an original inventory of large landslides (areas from 0.1 to 180 km²) established along the western flank of the Central Andes between latitudes ca. 15 and 20° S, and from the Pacific coast to the Altiplano. Based on manual mapping (using satellite images analysis, Google Earth and DEMs analysis) and a compilation of previous works, we inventoried more than a thousand large landslides in this region. We then statistically explored the database according to the landslides typology, size, abundance and relation to geologic, tectonic and climatic settings of the Central Western Andes in order to provide a first insight on their controlling factors. Landslide size-frequency distribution follows a power-law with an exponent of 2.31 ± 0.16 and a cut-off of 4.0 ± 1.9 km² showing a strong contribution of the largest landslides to the cumulated landslide area. We revealed a dominance of rockslide typology (86%) characterized by in-mass slides, the rest being rock-avalanche type (14%) marked by typical granular-flow morphologies. Combination of specific lithology and great local relief emerge as favorable conditioning factor for large landslide initiation, in particular in the case of river incisions though ignimbrites of the Paleogene-Neogene (Huaylillas Formation), concentrating >30% of the landslides. Moreover, landslide clusters tend to follow crustal faults networks suggesting a long-term control of tectonic activity. Most of the identified landslides are paleo events. We tentatively argue that their triggering could not have been possible in the current hyper-arid conditions of the Atacama Desert and its periphery. Future research providing dating on some of the landslide clusters identified in this study is needed to explore possible temporal correlations between periods of landslide activity and external seismic and/or climatic cycles.
... Moreover, since our 2013 publication, Peru's Institute of Geology, Mining, and Metallurgy (Instituto Geológico Minero y Metalúrgico, or INGEMMET), made available online a revised surficial geologic mapping of the study region (Lajo Soto et al., 2001a, 2001bSalas et al., 2003). Considered along with geologic mapping and dating efforts by tephrachronologists (Thouret et al., 2016(Thouret et al., , 2017, the new mapping and geochronologic data now permit a more complete understanding of the spatial extent and age of formation of the Alca obsidian source. At the same time, the patterned geochemical variation documented within the Alca source region has enabled artifacts of various Alca sub-sources to be identified via NAA, ED-XRF, and pXRF at archaeological sites of various time periods throughout Peru (Matsumoto et al., 2018;Hu and Shackley, 2018;Belisle et al., 2020). ...
... Subsequent fluvial incision of these fills left remnant hanging terraces along valley margins (e.g., at Cahuana, Fig. 2) and draped younger material on older slopes (Gregory-Wodzicki, 2000; Thouret et al., 2007). Failure of steep slopes and debris avalanches have occurred (Thouret et al., 2017), suggested by lobate scars within the Chococo valley east of the town of Alca (Fig. 2). These processes would have introduced younger pyroclastic material originating at higher elevations to the lower valley. ...
... However, the presence of non-devitrified, high-quality Alca obsidian within these ignimbrites suggests a younger age, lending support to the INGEMMET mapping here. A more recent map by Thouret et al. (2017) designates Condorsayhua within the Las Lomas PDC, which they date elsewhere on the plateau to ~1.56-1.26 Ma. ...
Article
The Alca obsidian source in southern Peru is one of the largest and most geochemically complex sources of volcanic glass in South America. Hunter-gatherers first discovered and used Alca obsidian for stone tools at the end of the Pleistocene. Alca later became one of the three most economically important and widely distributed sources of obsidian in the Central Andean region. Systematic mapping and geochemical characterization efforts spanning 20+ years have revealed an extensive high-elevation source region composed of six geographically and compositionally distinct sub-sources. Here we synthesize research documenting the 2000 km² spatial extent of the Alca obsidian source, and we present expanded geochemical datasets for six Alca sub-sources (n = 238 geologic samples) obtained using neutron activation analysis (NAA), laboratory x-ray fluorescence (XRF), and portable (p)XRF. Results for Alca and for six other major obsidian sources in the Peruvian Andes illustrate the efficacy of these techniques to discriminate all major Peruvian obsidian sources, including Alca sub-sources. Comprehensive compositional data from the Alca source area, examined against accumulating obsidian artifact datasets from throughout Peru, reveal past human use of various Alca sub-sources. These cases contribute fine-grained behavioral information, made possible by a complex obsidian source with geographically patterned geochemical variation and a >12,000-year sequence of human interaction with this geologic resource.
... Audin and Bechir, 2006;Crosta et al., 2014). From North to South, some impressive examples among others are: the Chuquibamba landslide -40 km 3 (Margirier et al., 2015;Thouret et al., 2017), the Caquilluco landslide -15 km 3 , the Lluta Landslide -26 km 3 (Wörner et al., 2002 ;Strasser and Schlunegger, 2005), the Minimini landslide -v>5 km 3 , the Latagualla landslide -5.4 km 3 (Pinto et al., 2008) or the Magnifico landslide -0.2 km 3 (Mather et al., 2014;Crosta et al., 2017). Only few of those giant landslides have been precisely dated (e.g. ...
... Crosta et al., 2014; J o u r n a l P r e -p r o o f al., 2017). In those canyons, the incision can locally excess 1500 meters (Thouret et al., 2017) suggesting that the topography is probably the first preconditioning factor for those giant gravitational failures. The same conclusions were made by Strasser and Schlunegger (2005), and Wörner et al. (2002) regarding the Lluta landslide. ...
... This way, while the western flank of the Andes remains hyper-arid, the upper catchments collect a significant amount of water, flowing then throughout the Cordillera toward the Pacific (Litty et al., 2017). This discharge has maintained a constant incision in the valleys thus contributing to maintain very steep canyon flanks and critical topographic wedges highly prone to large-scale landslide failure (Thouret et al., 2017). As it was globaly reported by Korup et al. (2007), and locally by Wörner et al. (2002) in the Lluta valley, this suggests that the critical relief (see also Blöthe et al., 2015) that can be close, or even beyond, to its proposed upper strength limit may be one of the primary factor controlling the development of large landslides in the Andean canyons. ...
Article
The central part of the Western Andes holds an exceptional concentration of giant paleolandslides involving very large volumes of rock material (v > km3). While those gravitational slope failures are interpreted consensually as an erosional response to the geodynamic activity of the Andes (relief formation and tectonic activity), the question of their triggering mechanisms remains enigmatic. To clarify the respective roles of climatic versus seismic forcing on the Andean landslides, new temporal constraints on paleo movements are essential. Here, we focus on one of those giant slope failures, the Aricota giant landslide that damned the Locumba valley in southern Peru. We conducted fieldwork, high-resolution DEM analysis and cosmogenic nuclide dating to decipher its development history and failure mechanisms. Our results point to the occurrence of two successive rockslide events. A giant failure mobilizing a rock volume of ca. 2 km3 first produced a dam at 17.9 ± 0.7 ka. Considering its height of ca. 600 m, the Aricota rockslide dam is one of the five largest landslide dams. At 12.1 ± 0.2 ka, a second event produced ca. 0.2 km3 of material, and the rock-avalanche debris spread out over the dam. As the chronology of those two events is pointing to the two main paleoclimatic pluvial periods in this region (Heinrich Stadial 1a and Younger Dryas), we favor the interpretation of a climatic forcing. At a regional scale, the concomitant aggradation of alluvial terraces and fan systems along the nearby valleys highlights higher regional erosion, sediments supply and mass-wasting events during those paleoprecipitation events and strengthens this conclusion.
... The Arma Formation appears primarily on the Cotahuasi quadrangle (Lajo Soto et al. 2001a) and it corresponds with ignimbrites containing the Alca obsidian mapped by Rademaker and team (2021, in press). Thouret et al. (2016Thouret et al. ( , 2017 use different naming conventions in their mapping and chronometric dating of the complex ignimbrite sequence in the Cotahuasi region. They designate the southern area of ignimbrites where Rademaker et al. (2021) mapped Alca obsidian as the Upper Sencca Formation, dated to∼2.2-1.8 mya (Thouret et al. 2016), and the northern area containing Alca obsidian bedrock as the Lomas ignimbrite. ...
... The Lomas is dated to ∼1.56-1.3 mya elsewhere on the Cotahuasi quadrangle (Thouret et al. 2017). Because Alca-1 obsidian crops out in these ignimbrite units, they should be equivalent. ...
Article
This article reports the identification of the Sayrosa Source, a minor geologic source of volcanic glass referred to Rare Type-3 obsidian in the 1977 pilot study by Burger and Asaro. Located only 25 km northeast of the major Alca-1 deposit, this source was exploited in prehispanic times despite the relatively small size of its nodules. Occasional flakes and bifaces of Sayrosa obsidian appear at archaeological sites in the puna of Chumbivilcas and the Cusco Valley probably as the by-product of llama caravans carrying other goods such as meat, wool, salt and Alca obsidian from the high grasslands of northern Arequipa to the agricultural communities of Cusco.
... The western flank of the Andes is carved by deep valleys and canyons related to a regional uplift and erosion (e.g. Thouret et al., 2017). The progressive onset of the Andean relief during the Miocene acted as an important topographic barrier prevented the crossing of cloud and precipitations from the Amazonian basin (Houston and Hartley, 2003). ...
Article
It is of major importance for Earth surface sciences to reconstruct denudation rates in the most precise and accurate way. For this, it can be useful to test on the same setting methods based on different assumptions, such as those relying on geomorphological and geochemical observations. Here, we use an exceptionally suited setting in the Locumba catchment (southwestern Peruvian Andes) that offers the unique opportunity to compare denudation rates derived from in situ cosmogenic 3 He and 10 Be with a geomorphological sediment budget integrated over the last 18 ka. The sediment budget is estimated by determining the volume of sediment trapped in the Aricota lake that formed 18 ka ago after the occurrence of a giant rockslide dam. We reconstructed the topography of the Locumba valley before the dam emplacement and established that the captured sediment volume is 0.8 ± 0.1 km 3. Considering that the lake-water output is restricted to seepage through the dam and that overflow above the dam never occurred, this volume correctly represents the sediment flux integrated over the last 18 ka. Integrating this volume over the upstream catchment area (∼1500 km 2), we derived a corresponding mean erosion rate of 30 ± 9 mm.ka −1. Fluvial sediments feeding the Aricota lake were sampled to derive denudation rates from in-situ cosmogenic 10 Be in the silicates and from in-situ cosmogenic 3 He in the ferromagnesian minerals. Cosmogenic nuclide denudation rates from the main stream are 30 ± 2, 33 ± 2, 21 ± 1 and 82 ± 5 mm.ka −1 for the 10 Be-quartz, the 10 Be-feldspar, the 3 He-amphibole and 3 He-pyroxene, respectively. The consistency between the cosmogenic nuclide denudation rates derived from 10 Be in the silicates and the erosion rate derived from our sediment budget shows that the 10 Be accurately estimates of the sediment flux. Additionally, this work provides the first successful application of 10 Be-feldspar nuclide-mineral pair to derive catchment-mean denudation rate and demonstrate that 10 Be-feldspar can thus be a good alternative in catchments dominated by volcanic rocks with no quartz. The discrepancies observed between the denudation rates derived from the 3 He-amphibole and 3 He-pyroxene couples require further studies.
... On the other hand, long-term denudation rates are often estimated by quantifying the differential erosion on bare-rock surfaces of known age (e.g., Bögli, 1980;Akerman, 1983;Lauritzen, 1990) or by measuring the concentration of cosmogenic radionuclides on exposed rocks (Stone et al., 1998;Matsushi et al., 2010;Xu et al., 2013;Ryb et al., 2014a;Krklec et al., 2018). Most cosmogenic nuclides (e.g., 10 Be and 26 Al) are investigated in quartz of silicate rocks (e.g., Riebe et al., 2001;Braucher et al., 2003;von Blanckenburg, 2006;Cossart et al., 2008;Thouret et al., 2017). Carbonate lithologies normally contain too little quartz for those isotopes to be studied, thus 36 Cl cosmogenic nuclides are used instead to discern the type of the weathering (Xu et al., 2013;Ryb et al., 2014a), calculate the age of exposed bedrock surfaces or sediments after geomorphic events (Merchel et al., 2014;Benedetti and van der Woerd, 2014;Krklec et al., 2018;Petit et al., 2019) and determine denudation rates (Godard et al., 2016;Avni et al., 2018;Ben-Asher et al., 2021). ...
Article
The development of a karst landscape results from complex interactions between lithology, climate, hydrology, soil, vegetation and tectonics. Weathering and erosion of carbonate rocks leads to denudation of karst landscapes. As dissolution of carbonate rocks is often considered to be the main process governing carbonate weathering, other processes are often overlooked. Here we present research done in the North Dalmatian Plain, a carbonate erosive surface located in the Dinaric karst region. Although the study site is composed of two different carbonate lithologies having different weathering style, there is no evident lithological impact on the topography of the erosive surface. Analyses of ³⁶Cl concentration were performed in ten proximal bedrock samples from both lithologies and resulted in long-term denudation rates ranging from 14.7 to 22.8 m/Ma. Since no statistical significance was found between samples from different lithologies (all samples belong to a single normal population) and they have the same geomorphological context and climate features, variable denudation rates are attributed to local (sample specific) differences. In the study site there are no large outstanding rock residuals or patches with deep soil profiles. Thus, to maintain the levelled erosive surface, local differential denudation rates have to vary with time. We hypothesize that lichens and pedogenic carbonates have a significant role in modulating local differences in denudation rates. Our study shows that even at outcrop scale those differences can be significant, and the study of sample populations is preferred to single or limited number of analyses. Thus, the long-term denudation rate of the North Dalmatian Plain, including its local variability is 18.91 ± 0.81 m/Ma. Despite classical studies on karst terrains assume that dissolution is the main process responsible for development of these landscapes, our research highlights the importance of physical weathering in combination with dissolution of carbonates as main controls on the denudation of karst landscapes.
... Ma) with outflow sheets underlying both LVC's. Paleomagnetic measurements point to sources likely located below these clusters Thouret et al., 2017;Mariño et al., 2020). We document in detail the stratigraphic sequence and construction of the Chachani large volcanic cluster based on analyzed magmatic compositions and new 40 Ar/ 39 Ar ages. ...
... Ma) with outflow sheets underlying both LVC's. Paleomagnetic measurements point to sources likely located below these clusters Thouret et al., 2017;Mariño et al., 2020). We document in detail the stratigraphic sequence and construction of the Chachani large volcanic cluster based on analyzed magmatic compositions and new 40 Ar/ 39 Ar ages. ...
Article
In the Central Andes, large (> 500 km²) and long-lived (1–5 Ma) volcanic clusters (LVCs) are less explored and their eruptive history and magmatic regimes less understood than smaller, short-lived (<0.5 Ma), individual stratocones. The Chachani-large volcanic cluster (C-LVC) sizeable volume (c. 290 km³) consists of twelve edifices forming the 1.06–0.64 Ma group of stratovolcanoes and the 0.46–0.05 Ma group of domes coulees and block-lava flow fields. Both groups overlie pre-Chachani lavas and tuffs 1.02–1.27 Ma, and together they have buried large nested craters or a caldera associated with the c. 1.62–1.66 Ma Arequipa Airport ignimbrite. The C-LVC evolved from: (i) homogeneous compositions of the pre-Chachani and Chachani basal eruptive units to (ii) relatively wide compositional variations (53–67 wt% SiO2) between mafic andesite and dacite at moderate eruptive rates (0.27–0.41 km³/ka) for the ‘Old Edifice’ group, and finally to (iii) narrower (57–64 wt% SiO2) andesitic compositions coinciding with extrusive activity at 2.5 times lower eruptive rates (0.12–0.15 km³/ka) for the ‘Young Edifice’ group. The large compositional variations in the Old Edifice group are related to strongly contrasting resident and recharge magma compositions of hybridized lavas. In contrast, the narrow compositional range and lower eruption rate during the second half of the C-LVC eruptive history represent a trend towards more homogeneous, andesitic magma composition with time. Mineral texture and compositional studies provide evidence for disequilibrium and magma mixing in the C-LVC shallow (5–20 km depth range) magma reservoirs. These temporal changes in magma composition document that the transcrustal magma systems of the C-LVC evolved and matured with time by a combination of processes: fractional crystallization, crustal contamination and magma mixing/mingling with variable rates of mafic recharge. This resulted in a shift in time to a steady state, monotonous (andesite) regime as a result of coupling between compositional parameters and thermal conditions, density constraints, and the viscosity/crystallinity of erupted magmas.
... The offshore deposition of tephra in forearc basins could, potentially, provide a more complete record of the explosive activity of the Central Andes compared with on-land outcrops. The upper Miocene Pisco Formation corresponds to a period characterized by intense volcanic activity in the Central Volcanic Zone (Tosdal et al. 1981;Thouret et al. 2007Thouret et al. , 2017Kay et al. 2010). In particular, the stratigraphic interval studied here (from 8·5 to 7 Ma), which temporally includes the activity of the Lower Barroso arc (Mamani et al. 2010), slightly precedes the 5-6 Ma peak in the presence of tephra in the marine record off the coast of Peru at 10 to 11°S, found during Ocean Drilling Program (ODP) Leg 201 by Hart & Miller (2006). ...
Conference Paper
The upper Miocene Pisco Formation (Peru) represents a world-known fossil Lagerstätte containing abundant and exceptionally well-preserved marine vertebrates. A detailed chronostratigraphic reconstruction is indispensable to study this fossil record and to understand the evolution of marine vertebrates. Recent work (Bianucci et al., 2016; Di Celma et al., 2016; Gariboldi et al., in press) in the area of the western Ica River Valley defined a detailed chronostratigraphic framework for the Pisco Formation, containing all the fossil vertebrates observed in the area. Such chronostratigraphic framework, based on new 40Ar/39Ar ages on biotite from tephra layers integrated with diatom biostratigraphy, implements previous scattered radiometric data (Brand et al., 2011; Esperante et al., 2015). Tephra layers representing primary air-fall deposition of volcanic ash from the Peruvian Andes volcanoes are very frequent in the Pisco Formation. Several of them do not show evidence of reworking or bioturbation. Due to their regional dispersal and to their geologically instantaneous deposition (Lowe, 2011), they provide the opportunity not only to date specific layers, when suitable for radiometric age determination, but also to correlate different localities, through the chemical fingerprinting of tephra. We collected more than 200 tephra layers from different localities in the Ica Desert along six measured stratigraphic sections. Based on the estimated stratigraphic position, we analyzed specific tephra layers through petrographic characterization, glass shard morphology, electron probe microanalyses of glass shards and, where present, biotite crystals. Despite some difficulties encountered, such as similar magma or mineral composition, local weathering, lack of record due to marine current transport and change in depositional environments among different localities, the correspondence of the obtained data allowed to verify correlations that were supposed during field work and to trace tephra layers from distant outcrop localities, allowing to refine the chronostratigraphy of the Pisco Formation in the western Ica River Valley. Bianucci G. et al (2016) Journal of Maps, 12: 1037–1046. Brand L.R. et al (2011) J. South Am. Earth Sci., 31: 414–425. Di Celma C. et al (2016) Journal of Maps, 12: 1020-1028. Esperante R. et al. (2015) Palaeogeography, Palaeoclimatology, Palaeoecology, 417: 337–370. Gariboldi K. et al (in press) Newsletters on Stratigraphy. Lowe D.J. (2011) Quaternary Geochronology, 6: 107-153.
... The western slopes of the Andes are cut by numerous river valleys, while in the mountains, the rivers flow in the deeply and narrowly cut gorges and ravines. On the coast, river valleys are separated only by relatively low ridges (400-1500 m a.s.l.) that descend to almost the shore of the Pacific Ocean itself (Thouret et al., 2007(Thouret et al., , 2017Kalicki and Kukulak, 2009;Steffen et al., 2010;Kukulak et al., 2016). ...
Article
The Lomas de Lachay are the first ridges of the western slope of the Andes, about 100 km northward from Lima. This study examines the structure and age of fluvial terraces in the valleys of different orders. Accumulation of alluvium took place during the periods ca. 57 ka, 24–15 ka and likely the Late Glacial/Early Holocene. These periods correlate with pluvial Minchin, Tauca, and probably Coipasa (when fluvial activity in some Peruvian valleys was also noted). These alluviation phases were separated by erosion and terrace formation. The progressive aridization of the climate caused LGM aeolian covers to appear (21–18 ka) for the first time. Wind-blown sands fossilized older forms of relief and covered the valley slopes in the border zone between the lomas and desert. In the Holocene period, three types of geosystems developed in line with different morphogenetic activity: Coastal Desert (typified by a prevalence of aeolian processes); Lomas (very stable, with a prevalence of chemical weathering – tafones); and Mountain Desert (typified by very active fluvial and slope systems during El Nino events). Settlement phases in Lomas de Lachay were related to a high frequency of ENSO, and intensive grazing in recent centuries caused the creation of terracettes and buried soils on the slopes.
... Between the 1950s and the 1990s, however, vast irrigation programmes started in the flat detritic plateaus surrounding those narrow valleys to supply new farming areas. Those deeply incised valleys present steep walls with more than 200 m denivelation, making them prone to landsliding, as shown by the giant palaeo-landslides visible in the geomorphology 13 , certainly dating back from the valley incision before the mid Pliocene 20 . We focus on this 2,000 km 2 area to study the impact of land-use change on the erosion of a desertic area over 40 yr (1978-2018). ...
Article
Full-text available
Intensification of agriculture leads to stress on the environment and subsequently can have strong societal and ecological impacts. In deserts, areas of very high sensitivity to land-use changes, these local-scale impacts are not well documented. On the arid southwestern coast of Peru, several vast irrigation programmes were developed in the 1950s on the flat detritic plateau surrounding narrow valleys to supply new farming areas. We document the long-term effects of irrigation on the erosion of arid deserts in the Vitor and Siguas valleys, south Peru, using 40 yr of satellite data. We demonstrate that irrigation initiated very large slow-moving landslides, affecting one-third of the valleys. Their kinematics present periods of quiescence and short periods of rapid activity, corresponding to landslide destabilization by their headscarp retrogression. This analysis suggests that the landslide motion continues long after their initiation by irrigations. Those landslides affect the fertile valley floors, leading to the destruction of villages and agricultural areas. We conclude that modern intensive farming can strongly impact traditional agriculture in desert areas where water management is particularly critical. Slow-moving landslides in two valleys in Peru were initiated by irrigation programmes in the region, suggest analyses of 40 years of satellite data.
... The offshore deposition of tephra in forearc basins could, potentially, provide a more complete record of the explosive activity of the Central Andes compared with on-land outcrops. The upper Miocene Pisco Formation corresponds to a period characterized by intense volcanic activity in the Central Volcanic Zone (Tosdal et al. 1981;Thouret et al. 2007Thouret et al. , 2017Kay et al. 2010). In particular, the stratigraphic interval studied here (from 8·5 to 7 Ma), which temporally includes the activity of the Lower Barroso arc (Mamani et al. 2010), slightly precedes the 5-6 Ma peak in the presence of tephra in the marine record off the coast of Peru at 10 to 11°S, found during Ocean Drilling Program (ODP) Leg 201 by Hart & Miller (2006). ...
Conference Paper
Tephra fingerprinting is a unique tool for reconstructing a high resolution stratigraphy (Lowe, 2011; Smith et al., 2011). In the upper Miocene succession of the Pisco Formation (East Pisco Basin, Peru) the presence of distal volcanic ashes from the Central Andes represents a great opportunity for dating and correlating stratigraphic sections at distant localities. The importance of a detailed chronostratigraphic reconstruction is given by to the paleontological significance of this formation, which hosts a globally renowned marine vertebrate Fossil-Lagerstätte (Lambert et al., 2010; Bianucci et al., 2016). For reaching this goal, 39Ar–40Ar dating and tephra fingerprinting were applied on ash layers. Regarding to 39Ar–40Ar dating, an essential part of our work were electron microprobe tests of stoichiometry and monomodality, so as to only date unaltered, homogeneous tephra. Despite the similar glass composition and mineral assemblage, together with the shallow marine depositional environment limiting tephra preservation, correlations between distant localities can be realized by fingerprinting tephra layers on the basis of petrographic and compositional investigations, grain-size analyses, and glass shard morphology. Major element composition of biotite proved to be a valuable tool for discriminating ash layers and correlating different stratigraphic sections located several kilometers apart from each other. This study, in part published this year on the Journal of the Geological Society (Bosio et al., 2019), highlights the applicability of tephra fingerprinting in tephra archives as old as the Miocene as well as in unfavorable shallow marine environments, and allows a great increase of the chronostratigraphic detail.
... The offshore deposition of tephra in forearc basins could, potentially, provide a more complete record of the explosive activity of the Central Andes compared with on-land outcrops. The upper Miocene Pisco Formation corresponds to a period characterized by intense volcanic activity in the Central Volcanic Zone (Tosdal et al. 1981;Thouret et al. 2007Thouret et al. , 2017Kay et al. 2010). In particular, the stratigraphic interval studied here (from 8·5 to 7 Ma), which temporally includes the activity of the Lower Barroso arc (Mamani et al. 2010), slightly precedes the 5-6 Ma peak in the presence of tephra in the marine record off the coast of Peru at 10 to 11°S, found during Ocean Drilling Program (ODP) Leg 201 by Hart & Miller (2006). ...
Article
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The contribution of landslides to the Quaternary evolution of reliefs is poorly documented in arid contexts. In southern Peru and northern Chile several massive landslides disrupt the arid western Andean front. The Chuquibamba landslide, located in southern Peru, belongs to this set of large landslides. In this area, the Incapuquio fault system captures the intermittent drainage network and localizes rotational landslides. Seismic activity is significant in this region with recurrent Mw = 9 subduction earthquakes, however none of the latest seismic events have triggered a major landslide. New terrestrial cosmogenic dating of the Chuquibamba landslide provides evidence that the last major gravitational mobilization of these rotational landslide deposits occurred at ∼ 102 ka, during the Ouki wet climatic event identified on the Altiplano between 120 and 98 ka. Our results suggest that wet events in the arid and fractured context of the Andean forearc induced these giant debris-flows. Finally, our study highlights the role of tectonics and climate on (i) the localization of large Andean landslides and on (ii) the long-term mass transfer to the trench along the arid Andean front.
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In the external forearc of southern Peru (Arequipa region), the sedimentary facies and the stratigraphic architecture of the Cenozoic Camaná Formation are presented in the context of tectono-eustatic controls. The Camaná Formation is defined as ∼500 m thick coarse-grained deltaic complex that accumulated in a fault-bounded elongated depression extending from the Coastal Cordillera in the east to the offshore Mollendo Basin in the west and likely up to the Peruvian Trench. Based on the analysis of facies associations, we propose a refined stratigraphic scheme of the Camaná Basin fill. The Camaná Formation was formerly divided into the Camaná “A” and Camaná “B” units (CamA and CamB, respectively). We reinterpret the stratigraphic position and the timing of the CamA to CamB boundary, and define three sub-units for CamA, i.e. sub-units A1, A2, and A3. Each depositional unit shows individual stacking patterns, which are linked with particular shoreline trajectories through time.
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High relief and steep rainfall gradients make the eastern flank of the northern Bolivian Andes an excellent location for deciphering the relative roles of tectonics and climate on erosion and landscape evolution. We seek to resolve the climate versus tectonics debate in this location by linking topographic analyses and erosion rate data with fluvial bedrock incision theory and numerical landscape evolution modeling. We find that patterns in the channel steepness index (channel slope normalized for drainage area) in both transverse channels that drain across the rainfall gradient through the driest and wettest parts of the landscapes, and frontal channels that drain only the wettest regions are indicative primarily of a gradient in rock uplift rate, although climate likely plays a secondary role in shaping these channel profiles. Previously published erosion rates from 23 watersheds vary with the proposed rock uplift gradient and inversely with rainfall rate, suggesting that increased rainfall is not driving increased rock uplift and erosion. The channel steepness index in an additional 35 tributary watersheds increases with the proposed rock uplift gradient. Simulations from a landscape evolution model that isolate the signatures of rainfall and uplift patterns on landscape morphology corroborate our interpretation that the morphology of this landscape is primarily controlled by a gradient in rock uplift rate, with rainfall rates playing a secondary role. Model results also suggest that the differences between channel steepness values in the transverse and frontal channels cannot be explained by the uplift and rainfall patterns alone. Differences in lithology may be contributing to the higher channel steepness values in the transverse channels, or the transverse channels may be affected by a transient oversteepening phenomenon seen in tools-and-cover river incision models. The conclusions are possible only after detailed comparisons among real and modeled rivers of different sizes that drain different locations. We present best practices for future studies that seek to resolve the relative imprint of rock uplift and rainfall on a landscape.
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Measured rates of river incision into bedrock are commonly interpreted as proxies for rates of rock uplift (see refs 1 and 2, for example) and indices of the strength of climatic forcing of erosion over time (see refs 3 and 4, for example). This approach implicitly assumes that river incision rates are in equilibrium with external forcings over a wide range of timescales. Here we directly test this assumption by examining the temporal scaling of bedrock river incision from 155 independent measurements of river incision compiled from 14 sites. Of these sites, 11 exhibit a negative power-law dependence of bedrock river incision rate on measurement interval, a relationship that is apparent over timescales of 10(4)-10(7) years and is independent of tectonic and geomorphic setting. Thus, like rates of sediment accumulation, rates of river incision into bedrock exhibit non-steady-state behaviour even over very long measurement intervals. Non-steady-state behaviour can be explained by episodic hiatuses in river incision triggered by alluvial deposition, if such hiatuses have a heavy-tailed length distribution. Regardless of its cause, the dependence of incision rate on measurement interval complicates efforts to infer tectonic or climatic forcing from changes in rates of river incision over time or from comparison of rates computed over different timescales.
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The western Andean mountain front forms the western edge of the central Andean Plateau. Between 18.5° and 22°S latitude, the mountain front has ~3000 m of relief over ~50 km horizontal distance that has developed in the absence of major local Neogene deformation. Models of the evolution of the plateau, as well as paleoaltimetry estimates, all call for continued large-magnitude uplift of the plateau surface into the late Miocene (i.e., younger than 10 Ma). Longitudinal river profiles from 20 catchments that drain the western Andean mountain front contain several streams with knickpoint-bounded segments that we use to reconstruct the history of post-10 Ma surface uplift of the western flank of the central Andean Plateau. The generation of knickpoints is attributed to tectonic processes and is not a consequence of base level change related to Pacific Ocean capture, eustatic change, or climate change as causes for creating the knickpoint-bounded stream segments observed. Minor valley-filling alluvial gravels intercalated with the 5.4 Ma Carcote ignimbrite suggest uplift related river incision was well under way by 5.4 Ma. The maximum age of river incision is provided by the regionally extensive, approximately 10 Ma El Diablo-Altos de Pica paleosurface. The river profiles reveal that relative surface uplift of at least1 km occurred after 10 Ma.
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Space geodetic estimates of the rate of Nazca-South America convergence and Nazca-Pacific spreading averaging over several years show that present day rates are significantly slower than the 3 million year average NUVEL-1A model. The implied rates of deceleration are consistent with longer term trends extending back to at least 20 Ma, about the time of initiation of Andes growth, and may reflect consequences of ongoing subduction and construction of the Andes, e.g., increased friction and viscous drag on the subducted slab as the leading edge of South America thickens.
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A large sedimentary forearc basin developed in Cenozoic times between the present-day Coastal Cordillera and the Western Cordillera of the Central Andes, called Moquegua basin in southern Peru. The basin is filled by Moquegua Group deposits (~50 to 4Ma) comprising mostly siliciclastic mudstones, sandstones and conglomerates as well as volcanic intercalations. Several facies changes both, along orogenic strike and through time, are described and have led to subdivision into four sedimentary units (Moquegua A, B, C and D). In this paper we present a refined stratigraphic scheme of the Moquegua Group combined with the first provenance analysis of the Moquegua basin based on (i) semi-quantitative analysis of heavy mineral abundance, (ii) electron microprobe (EMP) and laser ablation (LA) ICP-MS analyses of single detrital amphibole and Fe–Ti oxide grains, and (iii) comparative analysis of the different potential source rocks to clearly identify the most likely sources. Results allow us to reconstruct sediment provenance and to relate changes of the erosion-sedimentation system in the Moquegua basin to the evolution of the Andean orogen. At ~50 to ~40Ma the Moquegua basin was close to sea level and fed by low energy rivers transporting mainly metamorphic basement and Jurassic-Cretaceous sedimentary detritus from local and distal sources. The latter might be as far as the present Eastern Cordillera. From ~35Ma on the distal sediment sources were cut off by the uplift of the Altiplano and Eastern Cordillera leading to higher energy fluvial systems and increasing importance of local sources, especially the relevant volcanic arcs. From 25Ma on volcanic arc rocks became the predominant sources for Moquegua Group sediments. The 10Ma time lag observed between the onset of uplift-induced facies and provenance changes (at ~35Ma) and the onset of intense magmatic activity (at ~25Ma) suggests that magmatic addition was not the main driver for crustal thickening and uplift in the Central Andes during latest Eocene to Oligocene time.
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Compositional variations of Central Andean subduction-related igneous rocks reflect the plate-tectonic evolution of this active continental margin through time and space. In order to address the effect on magmatism of changing subduction geometry and crustal evolution of the upper continental plate during the Andean orogeny, we compiled more than 1500 major- and trace-element data points, and 650 Sr-, 610 Nd-, and 570 Pb-isotopic analyses of Mesozoic-Cenozoic (190-0 Ma) magmatic rocks in southern Peru and northern Chile (Central Andean orocline), mostly from new data and the literature. This data set documents compositional variations of magmas since Jurassic time, with a focus on the Neogene period, when major crustal thickening developed and its influence on magma composition was most pronounced. We relate the observed variations in Sr/Y, La/Yb, La/Sm, Sm/Yb, and Dy/Yb ratios, as well as in Sr-, Nd-, and Pb-isotopic ratios, to the crustal structure and evolution of the Central Andean orocline. In particular, the evolution of Dy/Yb and Sm/Yb ratios, which track the presence of the higher-pressure minerals amphibole and garnet, respectively, in the lower crust, documents that crustal thickness has grown through time. Spatial variations in trace elements and isotopic ratios further suggest that crustal domains of distinct composition and age have influenced magma composition through some assimilation. The crustal input in Quaternary magmas is quantified to have been between 7% and 18% by simple two-components mixing. When comparing our geochemical data set to the geological record of uplift and crustal thickening, we observe a correlation between the composition of magmatic rocks and the progression of Andean orogeny. In particular, our results support the interpretation that major crustal thickening and uplift were initiated in the mid-Oligocene (30 Ma) and that crustal thickness has kept increasing until present day. Our data do not support delamination as a general cause for major late Miocene uplift in the Central Andes and instead favor continued crustal thickening.
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The force balance in plate tectonics is fundamentally importantbut poorly known. Here, we show that two prominent and seeminglyunrelated observations---trench-parallel gravity anomaliesalong the Nazca-South America margin that coincide withthe rupture zones of great earthquakes, and a rapid slowdownof Nazca-South America convergence over the past 10 m.y.---providekey insights. Both result from rapid Miocene-Pliocene upliftof the Andes and provide quantitative measures of the magnitudeand distribution of plate coupling along the Nazca-SouthAmerica margin. We compute the plate-tectonic force budget usingglobal models of the faulted lithosphere coupled to high-resolutionmantle circulation models and find that Andean-related plate-marginforces are comparable to plate-driving forces from the mantle,and they have sufficient magnitude to account for pronouncedbathymetry variations along the trench. Our results suggestthat plate coupling, gravity anomalies, and bathymetry variationsalong a given trench are all controlled by long-term stressvariations in the upper portion of plate boundaries and thatan explicit budget of driving and resisting forces in platetectonics can be obtained. For the convergent margin consideredhere, spatial variations in the effective coefficient of frictionassociated with the distribution of lubricating sediments enteringthe trench are, by comparison, of minor importance.
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We address the question of the late Cenozoic geomorphological evolution of the central Chile Andes (33°–35°S), using uplift markers, river incision, previous and new ages of volcanic bodies, and new fission track ages. The uplift markers consist of relicts of high elevated peneplains that evidence >2 km of regional surface uplift lasting ∼2 Ma with variable amount along an E-W transect. The eastern Coastal Cordillera was uplifted 1.5–2.1 km at 33–34°S and 2.5 at 33°45′S and ∼1.5 km at 34°30′S. Erosional response to uplift was characterized by the retreat of a sharp knickpoint with celerities between 10 and 40 mm a−1. Extrapolation using a stream power law shows that uplift began shortly before 4 Ma or at 10.5–4.6 Ma (7.6 Ma central age) depending on the morphostructural units involved. The first alternative implies simultaneous uplift of the continental margin. The second model (the most reliable one) implies that the uplift affected together the eastern Coastal Cordillera and the Principal Cordillera, while the rest of the western fore arc subsided. This regional uplift can be mostly balanced by crustal thickening resulting from coeval shortening related to the out-of-sequence thrusting event in the Principal Cordillera and the uplift of the Frontal Cordillera. Simultaneously, emplacement of the southern edge of the flat slab subduction zone might have partially contributed to this uplift event.
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We explore canyon incision history of the western margin of the Andean (Altiplano-Puna) plateau in the central Andes as a proxy for surface uplift. (U-Th)/He apatite data show rapid cooling beginning at ca. 9 Ma and continuing to ca. 5.1 Ma in response to incision. A minimum of 1.0 km of incision took place during that interval. The youngest apatite date and a volcanic fl ow perched 125 m above the present valley fl oor dated at 2.261 ± 0.046 Ma (40Ar/39Ar) show that an additional ~1.4 km of incision occurred between ca. 5.1 and 2.3 Ma. Thus, we infer that a total of at least 2.4 km, or 75% of the present canyon depth was incised after ca. 9 Ma. (U-Th)/He zircon data collected along the same transect imply that the western margin of the plateau was warped upward into its present monoclinal form, rather than uplift being accommodated on major surface-breaking faults.
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Structural and thermochronologic studies of the western margin of the central Andean Plateau show changing styles of deformation through time that give insights into tectonic evolution. In southwest Peru, uplift of the plateau proceeded in several distinct phases. First, NW striking, NE dipping reverse faults accommodated uplift prior to ∼14–16 Ma. Subsequent uplift of the plateau relative to the piedmont (between the plateau and the Pacific Ocean) occurred between ∼14 and 2.2 Ma and was accommodated by NW striking, SW dipping normal faults and subparallel monoclinal folds. The youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block. Although the uplift magnitude associated with phase 1 is unknown, phases 2 and 3 resulted in at least 2.4–3.0 km of uplift. Up to 1 km of this may have occurred during phase 3. Geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift.
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The elevation of Earth's surface is among the most difficult environmental variables to reconstruct from the geological record. Here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters, constrained by measurements of abundances of 13C-18O bonds in soil carbonates. We use this approach to show that the Altiplano plateau in the Bolivian Andes rose at an average rate of 1.03 +/- 0.12 millimeters per year between approximately 10.3 and approximately 6.7 million years ago. This rate is consistent with the removal of dense lower crust and/or lithospheric mantle as the cause of elevation gain.
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The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dy-namics of terrane accretion along the Pacific. The Paleoproterozoic Arequipa terrane was trapped during collision between Laurentia and Amazonia in the Mesoproterozoic. Ultrahigh-temperature metamorphism correlates with the collapse of the Sunsás-Grenville orogen after ∼1000 Ma and is related to slab break-off and dispersal of Rodinia. The Antofalla terrane separated in the Neoproterozoic, forming the Puncoviscana basin. Its closure was coeval with the collision of the eastern Sierras Pampeanas. The rift-drift transitions of the early Paleozoic clastic platform showed a gradual younging to the north, in agreement with counterclockwise rotation based on paleomagnetic data of Antofalla. North of Arequipa arc magmatism and high-grade metamorphism are linked to collision of the Paracas terrane in the Ordovician, during the Famatinian orogeny in the Sierras Pampeanas. The early Paleozoic history of the Arequipa massif is explained by a backarc, which further south changed to open oceanic conditions and subsequent collision. The Antofalla terrane reaccreted to the continental margin by the late Ordovician. These accretions and subsequent separations during the Mesoproterozoic, Neoproterozoic–early Cambrian, and late Cambrian–middle Ordo-vician are explained by changes in absolute motion of the Gondwana supercontinent during plate global reorganization.
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1] Apatite and zircon (U-Th)/He ages from Ocoña canyon at the western margin of the Central Andean plateau record rock cooling histories induced by a major phase of canyon incision. We quantify the timing and magnitude of incision by integrating previously published ages from the valley bottom with 19 new sample ages from four valley wall transects. Interpretation of the incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to subduction and volcanism. Furthermore, the large magnitude of incision leads to additional three-dimensional variations in the thermal field. We address these complications with finite element thermal and thermochronometer age prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages. Comparison of 275 model simulations to observed cooling ages and regional heat flow determinations identify a best fit history with 0.2 km of incision in the forearc region prior to $14 Ma and up to 3.0 km of incision starting between 7 and 11 Ma. Incision starting at 7 Ma requires incision to end by $5.5 to 6 Ma. However, a 2.2 Ma age on a volcanic flow on the current valley floor and 5 Ma gravels on the uplifted piedmont surface together suggest that incision ended during the time span between 2.2 and 5 Ma. These additional constraints for incision end time lead to a range of best fit incision onset times between 8 and 11 Ma, which must coincide with or postdate surface uplift.
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A steep escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). Strong north–south and east–west precipitation gradients are exploited in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. Late Miocene tectonics uplifted the Meseta/Altiplano plateau (~3000 m a.s.l.), which is underlain by a series of Tertiary volcanic-volcanoclastic rocks. Streams on this plateau remain graded to the Late Miocene base level. Below the rim of the Meseta, streams have responded to this ramp uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present-day base level defined by the Pacific Ocean. It is found that the Tertiary units on the plateau function as cap-rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. 10Be-derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky−1 on the Meseta/Altiplano, irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky−1 to 250 mm ky−1 and correlate positively with precipitation rates, but show no strong correlation with hillslope angles or channel steepness. These relationships are explained by the presence of a cap-rock and climate-driven fluvial incision that steepens hillslopes to near-threshold conditions. Since escarpment retreat and the precipitation pattern were established at least in the Miocene, it is speculated that the present-day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau. Copyright
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We quantitatively analyse the spatial pattern of deformation partitioning and of temporal accumulation of deformation in the Central Andes (15–26° S) with the aim of identifying those mechanisms responsible for initiating and controlling Cenozoic plateau evolution in this region. Our results show that the differential velocity between upper plate velocity and oceanic plate slab rollback velocity is crucial for determining the amount and rate of shortening, as well as their lateral variability at the leading edge of the upper plate. This primary control is modulated by factors affecting the strength balance between the upper plate lithosphere and the Nazca/South American Plate interface. These factors particularly include a stage of reduced slab dip (33 to 20 Ma) that accelerated shortening and an earlier phase (45 to 33 Ma) of higher trenchward sediment flux that reduced coupling at the plate interface, resulting in slowed shortening and enhanced slab rollback. Because high sediment flux and transfer of convergence into upper plate shortening constitute a negative feedback, we suggest that interruption of this feedback is critical for sustaining high shortening transfer, as observed for the Andes. Although we show that climate trends have no influence on the evolution of the Central Andes, the position of this region in the global arid belt in a low erosion regime is the key that provides this interruption; it inhibits high sediment flux into the trench despite the formation of relief from ongoing shortening. Along-strike variations in Andean shortening are clearly related to changes of the above factors. The spatial pattern of distribution of deformation in the Central Andes, as well as the synchronization of fault systems and the total magnitude of shortening, was mainly controlled by large-scale, inherited upper plate features that constitute zones of weakness in the upper plate leading edge. In summary, only a very particular combination of parameters appears to be able to trigger plateau-style deformation at a convergent continental margin. The combination of these parameters (in particular, differential trench-upper plate velocity evolution, high plate interface coupling from low trench infill, and the lateral distribution of weak zones in the upper plate leading edge) was highly uncommon during the Phanerozoic. This led to very few plateau-style orogens at convergent margins like the Cenozoic Central Andes in South America or, possibly, the Laramide North American Cordillera.
Article
With an area exceeding 25 000 km² and volumes c. 5000 km³, south Peru hosts the Andes’ second largest Neogene ignimbrite field.We document the extent, stratigraphy and chronology of 12 ignimbrite sheets in the RíoOcoña–Cotahuasi–Marán and Colca deep canyons. Based on 74⁴⁰Ar/³⁹Ar and U/Pb age determinations, ignimbrite-forming episodes span 25 myr. Prior to 9 Ma, eight large-volume ignimbriteswere produced every 2.4 myr. After 9 Ma, average lulls between small- to moderate-volume ignimbrites decreased to 0.85 myr. The refined volcanic stratigraphy reveals three main features. (1) Larger volume ignimbrites were emplaced by punctuated flare-ups between 25 and 9 Ma during uplift of the Western Cordillera. (2) Numerous smaller ignimbrites were emplaced after 9 Ma as the ignimbrite production rate decreased threefold. This decrease may be due to the declining crustal melting rate, decreasing plate convergence rate after 9 Ma, ormoremagma stagnation in the shallow crust, which promoted the growth of composite cones. (3) Growth of two volcanic arcs has added twice as much volume (c. 53 km³ Ma⁻¹) to the Río Ocoña–Cotahuasi–Marán volcanic field than the ignimbrites after 2.27 Ma. Estimated linear arc magma output has, however, decreased twofold (0.15 – 0.08 km³ km⁻¹ Ma⁻¹) from the Early Quaternary to the Pleistocene–Holocene. © 2016, Journal of the Geological Society. All rights reserved.
Article
New Rb/Sr whole-rock data (approx 20 isochrons) and K/Ar data (approx 30 ages) are presented and interpreted within a complete geochronological review of the Coastal batholith. In particular the superunit concept of synchronous emplacement throughout the batholith is tested, in most cases with the data justifying observed stratigraphic order. Some superunits, however, exhibit considerable diachroneity, e.g. Santa Rosa (100-50 m.y.). The overall ages for the segments range approx 190-60 m.y. for Toquepala, approx 100-24 m.y. for Arequipa and approx 100-37 m.y. for Lima. Some superunits yield ages much younger than expected, e.g. the newly defined Linga-Arequipa (Toquepala segment). Initial 87Sr/ 86Sr ratios are mostly 0.7040-0.7043 for the Lima segment, indicating a uniform mantle source region, but range <0.708 in the Arequipa segment due to contamination from the Precambrian envelope. K/Ar ages of approx 10 m.y. for the Cordillera Blanca batholith and 11 O-isotope analyses are also reported.-R.J.P.
Article
A synthesis of the geochronology on basalt flows from the southern Uinkaret volcanic field indicates that basalts erupted within and flowed into Grand Canyon during four major episodes: 725-475 ka, 400-275 ka, 225-150 ka, and 150-75 ka. To extend the usefulness of these dates for understanding volcanic stratigraphy and lava dams in western Grand Canyon, we analyzed light detection and ranging (lidar) data to establish the elevations of the tops and bottoms of basalt-flow remnants along the river corridor. When projected onto a longitudinal river profile, these data show the original extent of now-dissected intracanyon flows and aid in correlation of flow remnants. Systematic variations in the elevation of flow bottoms across the Uinkaret fault block can be used to infer the geometry of a hanging-wall anticline that formed adjacent to the listric Toroweap fault. The 725-475 ka volcanism was most voluminous in the area of the Toroweap fault and produced dike-cored cinder cones on both rims and within the canyon itself. Mapping suggests that a composite volcanic edifice was created by numerous flows and cinder-cone fragments that intermittently filled the canyon. Reliable (40)Ar/(39)Ar dates were obtained from flows associated with this period of volcanism, including Lower Prospect, Upper Prospect, D-Dam, Black Ledge, and Toroweap. Large-volume eruptions helped to drive the far-traveled basalt flows (Black Ledge), which flowed down-canyon over 120 km. A second episode of volcanism, from 400 to 275 ka, was most voluminous along the Hurricane fault at river mile 187.5. This episode produced flow stacks that filled Whitmore Canyon and produced the 215-m-high Whitmore Dam, which may have also had a composite history. Basaltic river gravels on top of the Whitmore remnants have been interpreted as "outburst-flood deposit" but may alternatively represent periods when the river established itself atop the flows. Remnants near river level at miles 192 and 195, previously designated as Layered Diabase and Massive Diabase, have been shown by (40)Ar/(39)Ar dating to be correlative with dated Whitmore flow remnants, and they help document the downriver stepped geometry of the Whitmore Dam. The ca. 200 and 100 ka flows (previously mapped as Gray Ledge) were smaller flows that entered the canyon from the north rim between river mile 181 and Whitmore Canyon (river mile 187.5); they are concordant with dates on the Whitmore Cascade as well as other cascades found along this reach. The combined results suggest a new model for the spatial and temporal distribution of volcanism in Grand Canyon in which composite lava dams and edifices, that were generally leaky in proximal areas, were built from 725 to 475 ka near Toroweap fault and around 320 ka near Whitmore Canyon. New data on these and other episodes present a refined model for complex interactions of volcanism and fluvial processes in this classic locality. Available data suggest that the demise of these volcanic edifices may have involved either large outburst-flood events or normal fluvial deposition at times when the river was established on top of basalt flows.
Article
To determine the mechanisms responsible for mountain belt growth it is important to accurately establish the timing of surface uplift. Here we exploit the altitude control on the production rate of in situ cosmogenic nuclides to test the hypothesis that the Andes were uplifted in the late Miocene. High concentrations of in situ cosmogenic 3He (3Hecos) have previously been measured in alluvial boulders on the western flank of the Central Andes, northern Chile (Evenstar et al. 2009). These are consistent with deposition soon after formation of the surface (13-14 Ma). We have modelled the accumulation of cosmogenic 3He in several different surface uplift scenarios and compared them to the measured concentrations. The measured 3Hecos concentrations are too high to be produced by late Miocene uplift and imply that the western flank of the Andean Cordillera attained a substantial part of its current elevation prior to 14 million years ago.
Article
Extensive tracts of low-gradient topography in steep mountain ranges, either forming rangetop plateaus or terraced pediments on range flanks, are widely distributed in mountain belts around the world. Before the advent of plate tectonics, such populations of planar landforms were interpreted as vestiges of a post-orogenic raised peneplain, i.e., a low-gradient land surface resulting from the decay, during long intervals of base-level stability, of a previous mountain range that was subsequently raised once again to great elevations—thus forming a new mountain range. This two-stage model has been challenged by theories that advocate continuity in tectonic processes and more gradual changes in base level, and thus expect a more immediate and proportionate response of geomorphic systems. Here we present a global survey of erosion surfaces in mountain ranges and put existing theories and empirical evidence into a broad perspective calling for further research into the rates and regimes of long-term mountain evolution. The resulting library of case studies provides opportunities for comparative analysis and helps to classify the landform mosaics that are likely to arise from the interplay between (i) crustal regimes, which at convergent plate margins need be neither uniform nor steady at all times; (ii) radiation-driven and gravity-driven geomorphic regimes, which are mainly determined by crustal boundary conditions and climate; and (iii) paleogeography, through which clues about base-level changes can be obtained. We examine intracratonic and plate-margin settings, with examples from thin-skinned fold belts, thick-skinned fold belts, island-arc and other subduction-related settings, and bivergent collisional orogens. Results reveal that the existence of erosion surfaces is not a simple function of geodynamic setting. Although some erosion surfaces are pre-orogenic, evidence about their predominantly post-orogenic age is supported by apatite fission-track and helium rock-cooling signatures, stratigraphic age-bracketing, stream channel gradient patterns, and other direct or indirect dating criteria. It follows that many portions of mountain belts undergo unsteady, nonuniform post-orogenic landscape evolution trajectories, with intermittent opportunities for relief reduction. The resulting erosion surfaces remain preserved as signatures of transient landscape evolution regimes. We find that (i) occurrences of planar topography form populations of discrete, insular landscape units, only some of which could be interpreted as fragments of a fluvially dissected, and/or tectonically fragmented, regional peneplain. (ii) The post-orogenic time required for achieving advanced stages of relief reduction is variable, ranging from 3 to 70 Ma. (iii) Partly depending on whether the adjacent sedimentary basins were over- or underfilled, some erosion surfaces may have been controlled by raised base levels and may thus have formed at high elevations; however, in many cases they were disconnected from marine base levels by rapid surface uplift, thus acquiring their elevated positions in recent time. In some cases, subcrustal processes such as asthenospheric anomalies, and/or lithospheric slab tear or breakoff, explain extremely rapid, regional post-orogenic uplift. (iv) Overall, the conditions for achieving surface preservation in steep and tectonically active terrain are predictable but also quite varied and contingent on context.
Article
Rapid changes in slab geometry are typically associated with fragmentation of the subducted plate; however, continuous curvature of the slab is also possible. The transition from flat to normal subduction in southern Peru is one such geometrical change. The morphology of the subducted Nazca Plate along this transition is explored using intraslab earthquakes recorded by temporary regional seismic arrays. Observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest warping of the slab. Concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry. The presence of a thin ultra-slow velocity layer (USL) atop the horizontal Nazca slab is identified and located. The lateral extent of this USL is coincident with the margin of the projected linear continuation of the subducting Nazca Ridge, implying a causal relationship wherein increased hydration of the ridge results in the formation of the USL downdip. Waveform modelling of the 2-D structure in southern Peru using a finite-difference algorithm provides constraints on the velocity and geometry of the slab's seismic structure and confirms the absence of any tears in the slab. The seismicity and structural evidence suggests smooth contortion of the Nazca Plate along the transition from flat to normal subduction. The slab is estimated to have experienced 10 per cent strain in the along-strike direction across this transition.
Article
Northernmost Chile is home to a well-preserved disequilibrium landscape of great antiquity. Contrasting drainage patterns are developed on the western slope of the Altiplano plateau. The oldest of these patterns is a now-relict parallel-patterned drainage network. In places a younger pattern, comprising a series of deeply incised canyons, or quebradas, crosscuts the older parallel-patterned network. These canyons show strong evidence of a groundwater-sapping origin. We hypothesize that the initiation of the canyon network resulted from changes in the hydrological regime related to a drying out of climate of the forearc and to the uplift of the Altiplano plateau.
Article
The timing and magnitude of surface uplift provide important constraints on geodynamic models of orogen formation. Oxygen isotope (δ18O) and mass-47 isotopolog (Δ47) compositions from terrestrial carbonate sediments have been used with modern isotope and temperature lapse rates to infer past surface elevations of the Andes. However, these paleoaltimetry interpretations are contentious because variations in the oxygen isotope composition in meteoric water (δ18Op) are caused by changes in elevation (orographic) and regional climate. Here, we use a limited-domain isotope-tracking general circulation model to simulate changes in δ18Op and isotopic lapse rates in response to Andean surface uplift, and to re-evaluate δ18O and Δ47 changes in late Miocene carbonates previously associated with rapid Andean growth. Results indicate that Andean surface uplift leads to changes in low-level atmospheric circulation and an increase in precipitation along the eastern Andean flank which influences isotopic source and amount effects. Simulated changes in Andean δ18Op are not systematic with an increase in surface elevation, but are instead a function of orographic thresholds that abruptly change regional climate. A δ18Op decrease of > 5‰ over the central Andes and an increase in isotopic lapse rates (up to 0.8‰ km− 1) coincide with Andean surface uplift from 75 to 100% of modern elevation. These changes in the isotopic signature could account for the entire 3–4‰ δ18O depletion in late Miocene carbonate nodules, and suggest an Andean paleoelevation of ~ 3000 m (75% of modern elevations) before 10 Ma.
Article
Canyon incision into mountain topography is commonly used as a proxy for surface uplift driven by tectonic or geodynamic processes, but climatic changes can also instigate incision. The ~1250-kilometer (km)–long eastern margin of the Andean Plateau hosts a series of 1.5- to 2.5-km-deep canyons that cross major deformation zones. Using (U-Th)/He thermochronology, we document a transition from Miocene faulting to Pliocene canyon incision across the northeastern plateau margin. Regionally, widespread Pliocene incision into the eastern plateau margin is concurrent with a shift in global climate from early Pliocene warmth to late Pliocene cooling. Enhanced moisture transport onto the Andean Plateau driven by sea surface temperature changes during cooling is the likely pacemaker for canyon incision.
Article
The subduction of oceanic aseismic ridges, oceanic plateaus and seamount chains is a common process that takes place in a variety of tectonic settings and seems to coincide spatially and temporally with a gap of volcanic activity, shallow or even horizontal slab angles, enhanced seismic activity and various topographic features. In the present study we focus on these dynamic effects on the basis of 2D thermomechanical modelling incorporating effects of slab dehydration, mantle-wedge melting and surface topography development. In order to ascertain the impact of a moderate-size (200 × 18 km) aseismic ridge, 12 pairs of experiments (one for the case with a ridge, the other without) were carried out varying slab density and subducting-and overriding-plate velocities. By analysing pairs of experiments we conclude that subduction of a moderate-sized ridge does not typically result in strong slab flattening and related decrease of magmatic activity. This, in turn, suggests that, when slab flattening is indeed associated with the ridge subduction in nature, the slab itself should be in a nearly critical (i.e., transient from inclined to flat) state so that any local addition of positive buoyancy may strongly affect overall slab dynamics. Therefore, subducting ridges may serve as indicators of transient slab states in nature. Another important result from our study is the numerical quantification of strongly decreased magma production associated with flat slabs that may explain gaps in recent active volcanism at low-angle subduction margins. Lowering of magmatic rock production is caused by the absence of a hot mantle wedge above the flat slabs and does not directly depend on the mechanism responsible for the triggering of slab flattening. Finally we document several very distinct surface effects associated with the moderate-size ridge subduction such as local increase in elevation of overriding margin, enhancement of subduction erosion and landward trench displacement. Surface uplift may exceed the original ridge height due to additional uplift resulting from the overriding plate shortening. Topographic perturbations within the accretionary wedge domain are transient and have a tendency to relax after the ridge passes the trench. In contrast, the topographic high created in the continental portion of the overriding plate relaxes more slowly and may even be sustained for several millions of year after the ridge subduction. © 2009 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.
Article
In many regions of the world, deeply incised canyons demonstrate the net effects of physical processes active at Earth's surface in response to surface uplift. Low-temperature thermochronometry can constrain the timing and rates of bedrock incision, which is necessary for relating canyon incision to surface uplift over geological timescales. We analyzed four samples from the Cotahuasi–Ocoña canyon system in southwest Peru using 4He/3He thermochronometry and we present a new inversion model to identify continuous low-temperature cooling histories that are consistent with the observed data. Derived cooling histories limit the onset of fluvial incision to ∼13 to 8Ma. This is in agreement with previously reported interpretations based on a three-dimensional thermal model interpolation to a much larger set of conventional apatite (U–Th)/He ages. However, because 4He/3He thermochronometry constrains an independent cooling history for each sample, the results also permit testing of landscape-evolution models with greater spatial variability in exhumation compared to those models that can be tested by a geographically scattered set of conventional (U–Th)/He ages. The different cooling histories of the four samples require asynchronous incision along at least part of the canyon system that is best explained by headward propagation of fluvial incision by knickpoint migration.
Article
[1] River incision over geologic timescales can be a valuable indicator of regional surface uplift. However, extracting the timing of surface uplift relative to the onset of incision is complicated by changes in precipitation commensurate with topographic development. Evidence of large-scale river incision on the flanks of the Andean plateau has been cited in support of a rapid and recent surface uplift event. Recent climate modeling studies demonstrate large magnitudes of regional climate change associated with surface uplift of the Andes, which may have influenced river incision processes. Here we present an analysis of mid-Miocene (16 Ma) to present river incision of the southwest Peruvian Ocoña River. A Monte Carlo approach with ~1.6 × 105 different simulations is used to explore the range of surface uplift and paleoclimate histories that are compatible with the modern river profile and geological constraints on the incision timing and magnitude. A range of channel properties, including the erodibility coefficient and erosion threshold, are considered. Results indicate that deep canyon incision on the plateau flanks may not be as diagnostic of rapid surface uplift as previously thought. More specifically, the evolution of the Ocoña River is consistent with local plateau elevations of 1–3 km at 16 Ma and either a steady or punctuated uplift of 1.5–3.5 km since then. The range of acceptable uplift histories is sensitive to the long-term magnitude and temporal evolution of precipitation. Similar paleoprecipitation changes are expected to have modified river profile evolution elsewhere in the Andes.
Article
The elevation of the Andean Cordillera is a crucial boundary condi- tion for both climatic and tectonic studies. The Andes affect climate be- cause they form the only barrier to atmospheric circulation in the Southern Hemisphere, and they intrigue geologists because they have the highest plateau on Earth formed at a noncollisional plate margin, the Altiplano-Puna. Yet, until recently, few quantitative studies of their uplift history existed. This study presents both (1) a review of the quan- titative paleoelevation estimates that have been made for the Central and Colombian Andes and (2) an examination of the source and mag- nitude of error for each estimate. In the Central Andes, paleobotanical evidence suggests that the Altiplano-Puna had attained no more than a third of its modern elevation of 3700 m by 20 Ma and no more than half its modern elevation by 10.7 Ma. These data imply surface uplift on the order of 2300-3400 m since the late Miocene at uplift rates of 0.2-0.3 mm/yr. Paleobotanical and geomorphological data suggest a similar uplift history for the Eastern Cordillera—namely no more than half the modern elevation present by 10 Ma. No evidence exists for an exponential increase in uplift rate, as has been interpreted from fission- track data. These uplift rates mostly reflect mean surface uplift rather than rock uplift—that is, uplift of material points—because little dis- section of the western Eastern Cordillera has occurred south of lat 19°S and of the Altiplano-Puna. Thus, the Central Andean Plateau appears to be young. In the Colombian Andes, paleobotanical data imply rapid uplift of the Eastern Cordillera between 2 and 5 Ma at rates on the or- der of 0.6-3 mm/yr. However, some of this uplift is likely rock uplift due to erosion-driven isostatic rebound rather than mean surface uplift.
Article
The results of a paleomagnetic study along the fore arc of southern Peru (15–18!S) and northern Chile (18–19!S) are reported from middle to late Miocene ignimbrites (7 sites), late Oligocene to early Miocene ignimbrites (72 sites), Paleogene sediments (20 sites), and Mesozoic and Paleocene volcanics and intrusions (31 sites). Comparison of locality-mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5.2 ± 11.3! clockwise to 55.6 ± 7.0! counterclockwise. Spatially, the magnitude of counterclockwise rotations increases northward from !0! within the Chilean fore arc south of 18!300S to >45! north of 16!300S. In southern Peru, paleomagnetic rotations recorded in Paleogene red beds decrease from late Eocene to late Oligocene, whereas Miocene ignimbrites display no evidence of rotation. These new results confirm that the rotations recorded in the fore arc of southern Peru were acquired at least before !15 Ma, and probably before 25 Ma, and thus prior to the late Neogene shortening of the sub-Andes. The onset of major Andean shortening in the Eastern Cordillera during the latest Eocene–earliest Oligocene is interpreted to have triggered the bending of the Peruvian fore arc. The region of the Peruvian fore arc with the largest rotations appears to be the fore-arc counterpart of the Abancay deflection, a remarkable NE-SW offset in the axis of the Eastern Cordillera induced by a major regional preorogenic structure. We underline that the Abancay deflection should be seen as the northwestern boundary, and therefore as a key element, of the Bolivian Orocline
Article
Data obtained from low-temperature thermochronometers such as apatite fission-track and (U-Th)/He are combined with morphometric information extracted from digital elevation models. This combination shows several geomorphological effects that are caused by the migration of the Nazca Ridge along the Peruvian Coastal margin. Offshore, the depth of the deep-sea trench decreases by ∼1500 m where the Nazca Ridge collides with the continental South American Plate. Onshore the ridge causes an uplift of at least 800 m in the Coastal Cordillera. This uplift results in a westward shift of the coastline thereby focusing and increasing erosion in the uplifted areas. At the trailing edge, the shelf subsides and the coastline retreats eastwards, producing at least part of the indentation observed between Paita and Pisco. The Ridge acts therefore like a wave uplifting the Andean margin as it traverses inland and southwards leaving a clear fingerprint on the topographic evolution of the Peruvian coastal margin.
Article
1] The rapid rise of the central Andean plateau between $10 and 6.8 Ma implies that mantle lithosphere, including eclogitized lower crust, was removed from beneath the region in that time interval; we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred. Using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum (Rayleigh-Taylor instability), we place bounds on the average viscosity coefficient for central Andean lithosphere. When compared with laboratory measurements of flow laws for olivine and eclogite, the allowed range of viscosity coefficients yields bounds on the temperature of $500–800°C at the Moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than $50 MPa. This range of temperature is comparable with, if a slightly lower than, we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity. The mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part. Citation: Molnar, P., and C. N. Garzione (2007), Bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the Altiplano and Eastern Cordillera, Tectonics, 26, TC2013, doi:10.1029/ 2006TC001964.
Article
1] We present a quantitative reconstruction of uplift of the western flank of the Altiplano plateau (central Andes), one of the largest monoclines on the Earth, on the basis of an analysis of tectonic structures, syntectonic deposits, and geophysical data. Uplift occurred on a west vergent, slowly propagating system of high-angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge. The upper ramps determine local fold geometries while the lower ramp controls large-scale surface tilting and uplift. At 20°S, this fault system was active between $30 Ma and 5–10 Ma, with maximum shortening rates of 0.22 mm/yr between 17 and 10 Ma. It generated some 2600 m of surface uplift with only minor shortening of $3000 m. Its activity was largely synchronous to eruption of large-volume ignimbrites from a midcrustal source. Geophysical data indicate that the fault system localized deformation at the boundary between a cool, strong forearc crust and a presumably fluid-rich and/or partially molten zone underneath the plateau area. The systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank, with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments. Crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc, which only reacted with minor tilting. (2004), Uplift of the western Altiplano plateau: Evidence from the Precordillera between 20° and 21°S (northern Chile), Tectonics, 23, TC4004, doi:10.1029/ 2003TC001519.
Article
The Ayabacas Formation of southern Peru is an impressive unit formed by the giant submarine collapse of the mid-Cretaceous carbonate platform of the western Peru back-arc basin (WPBAB), near the Turonian–Coniacian transition (∼90–89 Ma). It extends along the southwestern edge of the Cordillera Oriental and throughout the Altiplano and Cordillera Occidental over >80 000 km2 in map view, and represents a volume of displaced sediments of >10 000 km3. The collapse occurred down the basin slope, i.e. toward the SW. Six zones are characterised on the basis of deformational facies, and a seventh corresponds to the northeastern ‘stable’ area (Zone 0). Zones 1–3 display increasing fragmentation from NE to SW, and are composed of limestone rafts and sheets embedded in a matrix of mainly red, partly calcareous and locally sandy, mudstones to siltstones. In contrast, in Zones 4 and 5 the unit consists only of displaced and stacked limestone masses forming a ‘sedimentary thrust and fold system’, with sizes increasing to the southwest. In Zone 6, the upper part of the limestone succession consists of rafts and sheets stacked over the regularly bedded lower part. The triggering of this extremely large mass wasting clearly ensued from slope creation, oversteepening and seismicity produced by extensional tectonic activity, as demonstrated by the observation of synsedimentary normal faults and related thickness variations. Other factors, such as pore pressure increases or lithification contrasts probably facilitated sliding. The key role of tectonics is strengthened by the specific relationships between the basin and collapse histories and two major fault systems that cross the study area. The Ayabacas collapse occurred at a turning point in the Central Andean evolution. Before the event, the back-arc basin had been essentially marine and deepened to the west, with little volcanic activity taking place at the arc. After the event, the back-arc was occupied by continental to near-continental environments, and was bounded to the southwest by a massive volcanic arc shedding debris and tuffs into the basin.
Article
Major slope failures are a significant degradational process at volcanoes. Slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years; the historic record provides evidence for at least six of these events in the past century. Several historic debris avalanches exceed 1 km3 in volume. Holocene avalanches an order of magnitude larger have traveled 50–100 km from the source volcano and affected areas of 500–1500 km2. Historic eruptions associated with major slope failures include those with a magmatic component (Bezymianny type) and those solely phreatic (Bandai type). The associated gravitational failures remove major segments of the volcanoes, creating massive horseshoe-shaped depressions commonly of caldera size. The paroxysmal phase of a Bezymianny-type eruption may include powerful lateral explosions and pumiceous pyroclastic flows; it is often followed by construction of lava dome or pyroclastic cone in the new crater. Bandai-type eruptions begin and end with the paroxysmal phase, during which slope failure removes a portion of the edifice. Massive volcanic landslides can also occur without related explosive eruptions, as at the Unzen volcano in 1792. The main potential hazards from these events derive from lateral blasts, the debris avalanche itself, and avalanche-induced tsunamis. Lateral blasts produced by sudden decompression of hydrothermal and/or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s−1. The ratio of area covered to distance traveled for the Mount St. Helens and Bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume. The potential for large-scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice, as at Mount St. Helens and Bezymianny. Debris avalanches can move faster than 100 ms−1 and travel tens of kilometers. When not confined by valley walls, avalanches can affect wide areas beyond the volcano's flanks. Tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions. The probable travel distance (L) of avalanches can be estimated by considering the potential vertical drop (H). Data from a catalog of around 200 debris avalanches indicates that the H/L rations for avalanches with volumes of 0.1–1 km3 average 0.13 and range 0.09–0.18; for avalanches exceeding 1 km3, H/L ratios average 0.09 and range 0.5–0.13. Large-scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts. The nature and duration of precursory activity vary widely, and the timing of slope faliure greatly affects the type of associated eruption. Bandai-type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity.
Article
Radiometric and geologic information indicate a complex history of Cenozoic volcanism and tectonism in the central Andes. K-Ar ages on silicic pyroclastic rocks demonstrate major volcanic activity in central and southern Peru, northern Chile, and adjacent areas during the Early and Middle Miocene, and provide additional evidence for volcanism during the Late Eocene. A provisional outline of tectonic and volcanic events in the Peruvian Andes during the Cenozoic includes: one or more pulses of igneous activity and intense deformation during the Paleocene and Eocene; a period of quiescence, lasting most of Oligocene time; reinception of tectonism and volcanism at the beginning of the Miocene; and a major pulse of deformation in the Middle Miocene accompanied and followed through the Pliocene by intense volcanism and plutonism. Reinception of igneous activity and tectonism at about the Oligocene-Miocene boundary, a feature recognized in other circum-Pacific regions, may reflect an increase in the rate of rotation of the Pacific plate relative to fixed or quasifixed mantle coordinates. Middle Miocene tectonism and latest Tertiary volcanism correlates with and probably is genetically related to the beginning of very rapid spreading at the East Pacific Rise.
Article
This paper analyses Late Cenozoic uplift in the Bolivian Andes, using the morphology of well preserved regional paleosurfaces in the Eastern Cordillera that define three axially draining braided river catchments that formed between ∼ 12 and ∼ 9 Ma. Rock uplift since the formation of the paleodrainage systems, which has been quantified using four different methods, is 1705 ± 695 m, with a mean erosion of 230 ± 90 m as a consequence of entrenchment of the drainage systems. The lack of faulting or tilting in the regions immediately farther west strongly suggest that rock uplift of the paleodrainage systems also extends to the western margin of the Eastern Cordillera. Balanced structural cross-sections require crustal deformation at depth beneath the Eastern Cordillera, in order to accommodate underthrusting of the Brazilian Shield beneath the thin-skinned fold and thrust belt on the eastern margin of the Bolivian Andes. In this case, the observed uplift of the Eastern Cordillera is easily explained by sliding up a ramp in the major decollement, dipping in the range 4°–16°W, as a consequence of the observed 60–110 km of shortening farther east, in the Subandean zone. Contemporaneous uplift of the western margin of the Eastern Cordillera is easily explained in terms of crustal thickening as a result of ductile squeezing in the lower crust accommodating at depth the Subandean shortening. It remains unclear how this uplift relates to that of the regions farther west, in the Altiplano and volcanic arc, except that uplift in the Eastern Cordillera coincides with a phase of intense shortening in the northern Altiplano, commencing at ∼ 9.5 Ma and continuing to ∼ 2.7 Ma and possibly younger.
Article
Silicic volcanism in the Andean Central Volcanic Zone (CVZ) produced one of the world's largest Neogene ignimbrite provinces. The largest and best-known CVZ ignimbrites are located on the Altiplano-Puna plateau north of 24 °S. Their compositions and huge erupted volumes suggest an origin by large-scale crustal melting, and present-day geophysical anomalies in this region suggest still active zones of partial melting in the middle crust. Farther south in the CVZ, the Cerro Galán complex erupted ignimbrites in the late Miocene and Pliocene that are quite similar in volume and composition to those from north of 24 °S and they have a similar origin. However, there are a great many other, smaller ignimbrites in the southern CVZ whose compositions and geodynamic significance are poorly known. These are the subject of this paper.
Article
The collision zone of the 200 km wide and 1.5 km high Nazca Ridge and the Peruvian segment of the convergent South American margin between 14°S and 17°S is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc. This is evident by a narrowing of the shelf, a westward shift of the coastline and the presence of marine terraces. As the Nazca Ridge is oblique with respect to both trench and convergence direction of the Nazca Plate, it migrates southward along the active plate boundary. For reconstructing the migration history of the Nazca Ridge, this study uses updated plate motion data, resulting from a revision of the geomagnetic time scale. The new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of ∼75 mm/a (before 10.8 Ma), ∼61 mm/a (10.8–4.9 Ma), and ∼43 mm/a (4.9 Ma to present). Intra-plate deformation associated with mountain building in the Peruvian Andes since the Miocene reduces the relative convergence rate between Nazca Plate and Peruvian forearc. Taking an intra-plate deformation at a rate of ∼10 mm/a, estimated from space-geodetic and geological data, into account, does not significantly reduce these lateral migration velocities. Constraining the length of the original Nazca Ridge by its conjugate feature on the Pacific Plate yields a length of 900 km for the subducted portion of the ridge. Using this constraint, ridge subduction began ∼11.2 Ma ago at 11°S. Therefore, the Nazca Ridge did not affect the northern sites of Ocean Drilling Program (ODP) Leg 112 located at 9°S. This is supported by benthic foraminiferal assemblages in ODP Leg 112 cores, indicating more than 1000 m of subsidence since at least Middle Miocene time, and by continuous shale deposition on the shelf from 18 to 7 Ma, recorded in the Ballena industrial well. At 11.5°S, the model predicts the passage of the ridge crest ∼9.5 Ma ago. This agrees with the sedimentary facies and benthic foraminiferal stratigraphy of ODP Leg 112 cores, which argue for deposition on the shelf in the Middle and Late Miocene with subsequent subsidence of a minimum of several hundred meters. Onshore at 12°S, the sedimentary record shows at least 500 m uplift prior to the end of the Miocene, also in agreement with the model.