Location of largest earthquake slip and fast rupture controlled by along-strike change in fault structural maturity due to fault growth

Earthquake slip distributions are asymmetric along strike, but the reasons for the asymmetry are unknown. We address this question by establishing empirical relations between earthquake slip profiles and fault properties. We analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults, in particular on the directions of their long-term lengthening. We find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long-term fault propagating tips, i.e., on the most mature half of the broken fault sections. Meanwhile, slip decreased linearly over most of the rupture length in the direction of long-term fault propagation, i.e., of decreasing structural maturity along strike. We suggest that this earthquake slip asymmetry is governed by along-strike changes in fault properties, including fault zone compliance and fault strength, induced by the evolution of off-fault damage, fault segmentation, and fault planarity with increasing structural maturity. We also find higher rupture speeds in more mature rupture sections, consistent with predicted effects of low-velocity damage zones on rupture dynamics. Since the direction(s) of long-term fault propagation can be determined from geological evidence, it might be possible to anticipate in which direction earthquake slip, once nucleated, may increase, accelerate, and possibly lead to a large earthquake. Our results could thus contribute to earthquake hazard assessment and Earthquake Early Warning

Using in situ Chlorine-36 cosmonuclide to recover past earthquake histories on limestone normal fault scarps: a reappraisal of methodology and interpretations

International audienceCosmic-ray exposure dating of preserved, seismically exhumed limestone normal fault scarps has been used to identify the last few major earthquakes on seismogenic faults and recover their ages and displacements through the modelling of the content of in situ [36Cl] cosmonuclide of the scarp rocks. However, previous studies neglected some parameters that contribute to 36Cl accumulation and the uncertainties on the inferred earthquake parameters were not discussed. To better constrain earthquake parameters and to explore the limits of this palaeoseismological method, we developed a Matlab® modelling code (provided in Supplementary information) that includes all the factors that may affect [36Cl] observed in seismically exhumed limestone fault scarp rocks. Through a series of synthetic profiles, we examine the effects of each factor on the resulting [36Cl], and quantify the uncertainties related to the variability of those factors. Those most affecting the concentrations are rock composition, site location, shielding resulting from the geometry of the fault scarp and associated colluvium, and scarp denudation. In addition, 36Cl production mechanisms and rates are still being refined, but the importance of these epistemic uncertainties is difficult to assess. We then examine how pre-exposure and exposure histories of fault-zone materials are expressed in [36Cl] profiles. We show that the 36Cl approach allows unambiguous discrimination of sporadic slip versus continuous creep on these faults. It allows identification of the large slip events that have contributed to the scarp exhumation, and provides their displacement with an uncertainty of +/- ~25 cm and their age with an uncertainty of +/-0.5-1.0 kyr. By contrast, the modelling cannot discriminate whether a slip event is a single event or is composed of multiple events made of temporally clustered smaller size events. As a result, the number of earthquakes identified is always a minimum, while the estimated displacements are maximum bounds and the ages the approximate times when a large earthquake or a cluster of smaller earthquakes have occurred. We applied our approach to a data set available on the Magnola normal fault, Central Italy, including new samples from the buried part of the scarp. Reprocessing of the data helps to refine the seismic history of the fault and quantify the uncertainties in the number of earthquakes, their ages and displacements. We find that the Magnola fault has ruptured during at least five large earthquakes or earthquake clusters in the last 7 ka, and may presently be in a phase of intense activity

Mecanismes de la deformation intracontinentale dans l'ouest des Etats-Unis

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Constraining erosion models using cosmogenic dating in Basin and Range, Nevada, USA

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Croissance et propagation des failles normales dans le rift d'Asal-Ghoubbet par datation cosmogénique 36CI

L'objectif de cette étude est de quantifier le fonctionnement des failles normales du rift d'Asal-Ghoubbet et de comprendre l interaction magamto-tectonique lors des processus de rifting. Notre approche consiste dans un premier temps, à estimer l âge d initiation de certaines failles qui structurent ce rift en déterminant, à partir d échantillons de basaltes, des durées d exposition au rayonnement cosmique en utilisant l isotope cosmogénique 36Cl. Pour se faire, nous avons donc mis en place un protocole chimique d extraction du chlore naturel et cosmogénique et réalisé un code numérique de calcul d âge d exposition. Les âges d exposition obtenus à la fois sur plagioclases, roche totale et sur la matrice de plusieurs échantillons, ainsi que la datation d une coulée basaltique reposant dur des calcaires d âges connus permettent de valider notre approche. Enfin, l estimation de l âge de mise en place d une coulée basaltique, antérieurement daté par K/Ar, montre que les processus érosifs doivent âtre pris en compte sur des larges gammes temporelles. Les mesures de décalages de différents marqueurs holocènes, réalisées à partir de photos numériques, permettent aussi d enregistrer le glissement vertical le long des ces failles pour, les derniers 6 ka partir de calcaires lacustres et pour le dernier millier d années à partir du liseré blanc. L ensemble de ces résultats montrent que l activité tectonique du rift d Asal s est initiée il y a 60 ka au Sud-Est du rift d Asal mais a réellement démarré il y a 35 ka, les failles atteignant leur extension maximale vers 20 ka. Ce résultat montre que ces failles se sont initiées sur toute leur longueur avant d'accumuler un déplacement vertical, ce qui va à l'encontre d'un développement auto-similaire simple. Les failles et le système de failles bordier Nord du rift se sont propagés globalement vers le Nord-Ouest rapidement (40-70 cm/an) avec des vitesses de glissement vertical élevées (10 mm/an) qui semblent avoir diminuées au fur et à mesure de leur développement. Enfin, le mode de croissance des failles ne semble pas non plus avoir changé sur des échelles de temps de quelques dizaines de ka. Dans un second temps, l analyse géochimique des coulées basaltiques qui remplissent le rift d Asal met en évidence qu il est l expression en surface d un méga-dyke lithosphérique d ~10 km de large et de ~60 km de profondeur qui induit un taux d amincissement de 6.25 cm/an sur une période de ~800 ka. Ces laves sont issues d une chambre magmatique peu profonde (~4 km). Ce réservoir magmatique a fonctionné par pulses entre au moins 300 et 66 ka. Le remplissage magmatique du rift correspondrait à un volume total de laves de l ordre de 20 km3 en 230 ka, mis en place à une vitesse moyenne de ~10^5 m3/an. La durée de ces pulses varie de quelques centaines d années à quelques 5 ka avec des temps de récurrence de quelques centaines d années à ~30 ka. Enfin, le développement des systèmes de failles au nord du rift serait responsable de la rotation antihoraire (plan horizontal) d une vingtaine de degrés de petits blocs de laves de longueur <= 2 à 8 km.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Stability of output fluxes of large rivers in South and East Asia during the last 2 million years: implications on floodplain processes

International audienceWe compare the present-day sediment discharge (solid phase) of some of the largest rivers in Asia to the average discharge deduced from the mass accumulated in several sedimentary basins during the Quaternary. There is a very good correlation, especially for the largest rivers: the Ganges–Brahmaputra, the Changjiang, the Huanghe and, to a lesser extent, the Indus and the Zhujiang. This suggests that present-day average discharge at the outlet has remained constant throughout the Quaternary at least for very large rivers (drainage area of the order of 105–106 km2). This, in turn, suggests either that continental denudation of large Asian catchments has remained on average constant, implying a strong tectonic control on erosion during the Quaternary, or that the river network has the ability to buffer changes in hillslope erosion or in sea-level in order to conserve the total discharge at the outlet. We show how this buffering capacity relies on the characteristic reaction time-scale of Asian alluvial plains (of the order of 105–6 years), that is, much higher than the time-scales of the Quaternary climate oscillations (of the order of 104 years). A short-term perturbation originating in hillslopes will be diluted by the floodplain. At the outlet the signal should have a longer time span and a smaller amplitude. In the same manner, an alluvial plain should not instantaneously react to a 104-year sea-level drop because of its inertia. Along with long-term tectonic control we infer this buffering to be the main cause for the average constancy of sediment yield of large Asian rivers during the Quaternary

Evolution morphologique et tectonique récente des marges NE et SE du plateau tibétain (lien avec la dynamique des grands fleuves)

PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Dynamique des rivières en tresses

PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Petrological constraints on melt generation beneath the Asal Rift (Djibouti) using quaternary basalts

International audienceThe temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 kyr). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y, and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na8.0 and Fe8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40°C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr-1, a value close to the mean spreading rate (2.9 ± 0.2 cm yr-1) over the last 620 kyr

Mass accumulation rates in Asia during the Cenozoic

International audienceThis work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in Asia since the beginning of the Cenozoic, ≈ 66 Ma. The estimates were derived from isopach maps, cross-sections and drill holes or stratigraphic columns assuming regional similarity of the strata. Average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods: Palaeocene ( ≈ 66-58 Ma), Eocene ( ≈ 58-37 Ma), Oligocene( ≈ 37-30 and 30-24 Ma), Miocene ( ≈ 24-17, 17-11 and 11-5 Ma), Pliocene ( ≈ 5-2 Ma) and Quaternary ( ≈ 2-0 Ma). These rates shed new light on the geological history of Asia since the onset of the collision of India with Asia ( ≈ 50 Ma). The overall average accumulation rates curve for Asian sedimentary basins since the beginning of the Tertiary shows an exponential form with slow accumulation rates (less than 0.5 × 106 km3 Myr− 1) until the beginning of the Oligocene, more than 15 Myr after the onset of the collision. From the Oligocene onwards rates increase quickly in an exponential manner, reaching their maximum values in the Quaternary (more than 1.5 × 106 km3 Myr− 1). From these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the India-Eurasia collision history. At smaller scales one may distinguish between independent histories at the subcontinental and basin scales. This permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of Asia during the Cenozoic