Using 10Be cosmogenic surface exposure dating to determine the evolution of the Purgatorio active fault in the Andean forearc, southern Peru

Active transpressive deformation has been occurring along the Andean hyperarid forearc for the last 3 Myrs but many of these faults are still not described even if able to produce large damaging earthquakes. Active faulting along the northern part of the Arica Bend can be recognized due to the presence of well-preserved and sharp fault scarps indicating recent surface slip. During the Mio-Pliocene, deposition within the forearc continental basins resulted in the formation of vast fan deposits and conglomerates of the Moquegua Formation, which can be considered as bedrock in this exposure study (~45-4 Ma; Tosdal et al., 1984; Sebrier et al., 1988a; Roperch et al., 2006). The typical vertical Purgatorio fault scarps offset both the Moquegua bedrock and several younger geomorphic features associated with <300kyrs climatic and 400 years old volcanic extreme events. This study focus on quantifying slip rate variations in time along a 5-meters high vertical fault scarp to understand how the fault is evolving. These results are achieved via surface exposure dating of the sampled seismically broken cobbolds of the Moquegua formation outcroping vertically along the fault scarp. These samples are well-suited to the application of in situ produced cosmogenic radionuclides for surface exposure dating, as the hyperarid region has extremely low erosion rates. We sampled the scarp away from any significant drainage so as to avoid possibly disturbed areas. The sampling did involve extracting quarzite conglomeratic material along the bedrock scarp and on the upper surrounding crests. The aim has been to measure Berylium-20 TCN (Terrestrial in situ Cosmogenic Nuclides) concentrations to determine exposure age as a function of height on the scarp. This has been successfully employed on one scarp in Italy based on Chlorine-36 TCN (Palumbo et al., 2004). However, slow faults behaviour remains unclear and more contributions are needed. Quaternary activity of the Purgatorio fault system was evidenced by Hall et al. (2008). They highlighted a vertical offset of about ~100 m for a pediment surface intercepted by the fault, and dated at ~280 ka. Considering that the pediment surface is horizontal, this would gave a maximum of ~0.3 mm/yr of vertical deformation since 280 ka. Our new data provide evidences of constant activity of the fault during the Holocene with a mean vertical motion of 2 ± 1 mm/ yr. These news results strengthen the idea that the Andean forearc is still submitted to contratile deformation, bring additional knowledge on the structural model of the area, and raise the question of the local seismological hazard

Uplift of the Bolivian orocline coastal areas based on geomorphologic evolution of marine terraces and abrasion surfaces: preliminary results

The southern Pacific coast morphology and especially the presence of marine surfaces gives information on the dynamics of Andean forearc evolution from the Neogene. Along most of the Southern Peru and Northern Chilean coasts, discontinuous uplifts are recorded by marine terraces and marine abrasion surfaces; they have thus, preserved a record of eustatic sea level changes and the uplift history of the coastal area in the Andean forearc. One approach to study the tectonic history of the Andean forearc is to identify its effects in marine sedimentation or erosion patterns along the coastal area. To investigate these processes, the Neogene marine formations are studied in various coastal sections either in southern Peru, at Chala (15°50'S) and Ilo (17°32'S-17°48'S), situated above a steep subduction segment and at San Juan de Marcona (15°20'S), situated above the southern part of the Nazca ridge; or in Chile, from Tongoy (30°15'S) to Los Vilos (31°55'S), situated above a flat subduction segment (Fig.1). We chose various sites from each branch of the Arica bend in order to sample possibly different time spans during the Neogene and different response of the continental plate to the subduction process. Various studies were already undertaken on such problems either in Peru or Chile but mainly leaded to the datation of the 5th isotopic stage. So, differential GPS and cosmogenic datations are pursued in order to propose robust ages on these sites and subtract the effects of eustatic sea-level changes from local curves, identifying tectonic uplifts

Compressive active fault systems along the Central Andean piedmont

It’s now established that Andean forearc is not concentrating as much tectonic shortening as the foreland since Middle Miocene. GPS measurements are neither available to inform on the long-term deformation across the Andes in Peru and anyway rather describe the elastic response of the Andean forearc to the Nasca-South American Plate convergence. Few neotectonic studies focuses on the Western side of the Andes and little is known about the active deformation in the Central Andes Pacific lowlands (Sébrier et al., 1988). Recent publications mainly improved the description of geomorphic surfaces (Thouret et al. 2007) and cosmogenic dating of the latter show much younger ones than expected (Hall et al., 2008). The topographic gradient on the western side of the Peruvian Andes is quite high as the trench (-7000m) lies only 200km away from the highest point (6000m). Moreover, authors still question the fact that the Andes build through a giant focused monocline or normal fault and demonstrate doing so the need of further mapping of the fault systems on the western side of the Central Andes (Schildgen et al., 2007)

Dynamique du soulèvement côtier Pléistocène des Andes centrales : Etude de l'évolution géomorphologique et datations (10Be) de séquences de terrasses marines (Sud Pérou - Nord Chili).

Geomorphic study and dating of marine terrace sequences along the coastal part of the Central Andean forearc evidence longitudinal and temporal variations in the response of the South American plate to the Nazca plate subduction during Pleistocene. The average uplift rates vary between 229 ± 40 (17.8°S) and 696 ± 53 mm/ka (15.33°S) along the coast. Rapid uplift periods alternate with slower uplift during the Pleistocene times. The morphologic evolution of the forearc is directly linked to active tectonic processes occurring along the subduction zone. Below a threshold in the coast-trench distance of about 100-110 km, the marine terraces are preserved and record the highest uplift rates of the whole Andean coastal front. The episodic uplift would be a result of the seismogenic zone surface variations during the Pleistocene and the related variation in the coast-trench distance.L'étude géomorphologique et la datation de séquences de terrasses marines, le long de la côte des Andes centrales, a permis de mettre en évidence une variabilité longitudinale et temporelle de la réponse de la plaque sud-américaine à la subduction de la plaque Nazca, au cours du Pléistocène. Les vitesses de soulèvement moyennes, déduites des âges 10Be, varient de 229 ± 40 (17,8°S) à 696 ± 53 mm/ka (15,33°S) le long de la côte. Des périodes de soulèvement rapide alternent avec des périodes de soulèvement plus lent au cours du Pléistocène. L'évolution morphologique de l'avant-arc est contrôlée par des processus tectoniques actifs liés à la zone de subduction. En-dessous d'une valeur seuil de 100-110 km de distance fosse-côte, les terrasses marines sont préservées et enregistrent les plus forts taux de soulèvement de la côte. Le soulèvement épisodique résulterait d'une variation de la surface de la zone sismogénique au cours du Pléistocène, en relation avec la variation de la distance fosse-côte

Dynamique du soulèvement côtier pléistocène des Andes centrales (étude de l'évolution géomorphologique et datations (10Be) de séquences de terrasses marines (Sud Pérou - Nord Chili))

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Subduction versus crustal tectonics impact on southern ecuadorian margin uplift: quantification of uplift rates, modelling of marine terraces, and interseismic coupling pattern

International audienc

Geometry and kinematic evolution of a long-living foreland structure inferred from field data and cross section balancing, the Sainte-Victoire System, Provence, France

International audienceThe Sainte-Victoire System (SVS) is a key area to understand how the shortening is accommodated in outer foreland of the Pyrenean-Provence orogen between Late Cretaceous and Eocene. Structural data, growth strata and fault slip analysis, and four balanced cross sections are used to decipher the along-strike geometry, deformational characteristics and kinematics of the SVS. The SVS is divided into two structural domains separated by a regional relay zone: the eastern domain is governed by a N-vergent thin-skinned tectonic style above Triassic series and the western domain, by a mixed S-vergent thick- and thin-skinned tectonic style with tectonic inversion of Late Paleozoic-Triassic half grabens. Growth strata indicate that the eastern SVS grew during Danian as a result of shortening transfer from the southern Arc Basin. In contrast, the western SVS is an independent structure which has recorded the early stage (∼83 Ma) of shortening and focused continuous deformation during ∼40 Myr. The shallow N-S shortening is ∼5 km (∼25%) and ∼8 km (∼34%) in the western and eastern SVS, respectively. At a regional scale, the tectonic inversion of the SVS and the Arc Basin recorded a deep shortening of the order of 15-18 km (∼34%). Although the shortening magnitude of the SVS remains small, other structures similar to the SVS were synchronously active across foreland basin, suggesting a significant amount of cumulated shortening. This outer foreland shortening may account for a non-negligible amount of deformation at the Pyrenean-Provence orogen scale

Interseismic Coupling and Quaternary Coastal Tectonics along the Andean Megathrust

International audienceThe Andean margin is one of the most active subduction zone worldwide (e.g., the1960 Mw 9.5 Chile or the Mw 8.8 1906 Ecuador earthquakes). We here proposethat the interseismic coupling along the Andean megathrust relates to spatialvariations along the interface as well as the fore-arc geology and inherited faultzones. For such matter, we compiled information on the extent of earthquakeruptures for the last 500 years, geodetic data and uplift rates derived from marineterraces from Ecuador to Chile. First, we show that 1) the coastline geometry, characterized by the distance between the coast and the trench, 2) the latitudinalvariations of long-term uplift rates, 3) and the spatial pattern of interseismic couplingare correlated. In addition, the seafloor roughness of the subducting plate isusually thought to be a cause of segmentation along subduction zones. But, afterdiscussing the role of inherited structures within the upper plate to the megathrustsegmentation in Ilo area during the Arequipa earthquake, we suggested thatthe continental structure itself may exert some feedback control on the segmentationand thus participate in the rupture pattern of major subduction earthquakes.This hypothesis tends to correlate with similar observations on crustal faultingin Pichelemu area in Chile. Finally, it’s now widely accepted that paleotsunamideposits can be used as the primary sources of information on past large megathrustevents. In southern Ecuador and northern Peru, the low coupling areas suggestrather rare tsunamigenic events and sedimentary evidences represent a usefultool to extend the cataloging of large earthquakes in the past thousand years. Alltogether these multidisciplinary approaches suggest that the spatial variationsof frictional properties along the megathrust dictate the geomorphology of thecoastal zone and can inform on the extent of seismic ruptures along strike

Interseismic Coupling and Quaternary Coastal Tectonics along the Andean Megathrust

International audienceThe Andean margin is one of the most active subduction zone worldwide (e.g., the1960 Mw 9.5 Chile or the Mw 8.8 1906 Ecuador earthquakes). We here proposethat the interseismic coupling along the Andean megathrust relates to spatialvariations along the interface as well as the fore-arc geology and inherited faultzones. For such matter, we compiled information on the extent of earthquakeruptures for the last 500 years, geodetic data and uplift rates derived from marineterraces from Ecuador to Chile. First, we show that 1) the coastline geometry, characterized by the distance between the coast and the trench, 2) the latitudinalvariations of long-term uplift rates, 3) and the spatial pattern of interseismic couplingare correlated. In addition, the seafloor roughness of the subducting plate isusually thought to be a cause of segmentation along subduction zones. But, afterdiscussing the role of inherited structures within the upper plate to the megathrustsegmentation in Ilo area during the Arequipa earthquake, we suggested thatthe continental structure itself may exert some feedback control on the segmentationand thus participate in the rupture pattern of major subduction earthquakes.This hypothesis tends to correlate with similar observations on crustal faultingin Pichelemu area in Chile. Finally, it’s now widely accepted that paleotsunamideposits can be used as the primary sources of information on past large megathrustevents. In southern Ecuador and northern Peru, the low coupling areas suggestrather rare tsunamigenic events and sedimentary evidences represent a usefultool to extend the cataloging of large earthquakes in the past thousand years. Alltogether these multidisciplinary approaches suggest that the spatial variationsof frictional properties along the megathrust dictate the geomorphology of thecoastal zone and can inform on the extent of seismic ruptures along strike