Incremental growth of normal faults: Insights from a laser-equipped analog experiment

International audienceWe conducted a laser-equipped analog experiment aimed at quasi-continuously monitoring the growth of a dense population of normal faults in homogeneous conditions. To further understand the way geological faults progressively gain in slip and length as they accumulate more strain, we measured with great precision the incremental slip and length changes that the analog faults sustain as they grow. These measurements show that the analog faults share common features with the natural ones. In particular, during their growth, the faults develop and maintain cumulative slip profiles that are generally triangular and asymmetric. The growth takes place through two distinct phases: an initial, short period of rapid lateral lengthening, followed by a longer phase of slip accumulation with little or no lengthening. The incremental slip is found to be highly variable in both space (along the faults) and time, resulting in variable slip rates. In particular, ‘short- and long-term' slip rates are markedly different. We also find that slip measurements at local points on fault traces do not contain clear information on the slip increment repeat mode. Finally, while the fault growth process is highly heterogeneous when considered at the scale of a few slip events, it appears homogeneous and self-similar at longer time scales which integrate many slip increments. This is likely to be the result of a feedback between stress heterogeneities and slip development. The long-term scale homogeneity also implies that the long-term faulting process is primarily insensitive to the short-term heterogeneities that are rapidly smoothed or redistributed. We propose a new conceptual scenario of fault growth that integrates the above observations and we suggest that faults grow in a bimodal way as a result of a self-driven and self-sustaining process

Western European Populations of the Ichneumonid Wasp Hyposoter didymator Belong to a Single Taxon

Hyposoter didymator (Hymenoptera, Ichneumonidae) is a generalist solitary endoparasitoid of noctuid larvae. In the present work, we tested whether populations of H. didymator were divided in several genetically distinct taxa as described for many other generalist parasitoid species, and whether differences in H. didymator parasitism rates were explained by the insect host species and/or by the plant on which these hosts were feeding on. The genetic analysis of natural populations collected in different regions in France and Spain on seven different insect hosts and seven different host plants (775 individuals) showed that H. didymator populations belong to a unique single taxon. However, H. didymator seems to be somewhat specialized. Indeed, in the fields it more often parasitized Helicoverpa armigera compared to the other host species collected in the present work. Also, H. didymator parasitism rates in field conditions and semi-field experimental studies were dependent on the host plants on which H. armigera larvae are feeding. Still, H. didymator can occur occasionally on non-preferred noctuid species. One hypothesis explaining the ability of H. didymator to switch hosts in natura could be related to fluctuating densities of the preferred host over the year; this strategy would allow the parasitoid to avoid seasonal population collapses

Stratigraphic architecture and fault offsets of alluvial terraces at Te Marua, Wellington fault, New Zealand, revealed by pseudo-3D GPR investigation,

International audiencePast earthquake slips on faults are commonly determined by measuring morphological offsets at current ground surface. Because those offsets might not always be well preserved, we examine whether the first 10 m below ground surface contains relevant information to complement them. We focus on the Te Marua site, New Zealand, where 11 alluvial terraces have been dextrally offset by the Wellington fault. We investigated the site using pseudo-3D Ground Penetrating Radar and also produced a high-resolution digital elevation model (DEM) of the zone to constrain the surface slip record. The GPR data reveal additional information: (1) they image the 3D stratigraphic architecture of the seven youngest terraces and show that they are strath terraces carved into graywacke bedrock. Each strath surface is overlain by 3-5 m of horizontally bedded gravel sheets, including two pronounced and traceable reflectors; (2) thanks to the multilayer architecture, terrace risers and channels are imaged at three depths and their lateral offsets can be measured three to four times, constraining respective offsets and their uncertainties more reliably; and (3) the offsets are better preserved in the subsurface than at the ground surface, likely due to subsequent erosion-deposition on the latter. From surface and subsurface data, we infer that Te Marua has recorded six cumulative offsets of 2.9, 7.6, 18, 23.2, 26, and 31 m (± 1-2 m). Large earthquakes on southern Wellington fault might produce 3-5 m of slip, slightly less than previously proposed. Pseudo-3D GPR thus provides a novel paleoseismological tool to complement and refine surface investigation

Strain partitioning in an accretionary wedge, in oblique convergence : analogue modelling

Bulletins Société Géologique de France, v. 173, n. 1, p. 17-24 [428], 2002International audienceIn subduction zones undergoing oblique convergence, strain partitioning is often expressed by an importantdeformation inducing strike-slip faulting. In accretionary wedges, parameters such as obliquity of the convergence andfriction at the bottom of the wedge play an important role in the strain partitioning. The impact of these parameters isstudied using sandbox experiments. Two backstop geometries have been designed to account for different geologicalsettings. These experiments show that the wedge taper remains constant and close to αcoulomb for variable obliquities.Measurements of critical tapers on the models suggest that the Coulomb wedge theory cannot be simply applied to determine parameters on wedges developed under oblique convergence. Parameters deduced from this theory are validonly when measured in the direction parallel to the convergence. In addition, the partition degree increases with theobliquity of the convergence, and strain partitioning occurs independently of the basal friction. We remark that themodel morphology changes when an obliquity value, mainly, is exceeded. A transcurrent structure develops. The models show that oblique structures located above the velocity discontinuity are associated with strike-slip faults. Similarstructures have been observed within the Hikurangi accretionary wedge (New Zealand).Au niveau des zones de subduction oblique, la partition du mouvement se traduit souvent par une déformation importante caractérisée par la coexistence de chevauchements avec de grandes failles décrochantes. Dans unprisme d’accrétion, des paramètres comme l’obliquité de la convergence et la friction à la base du prisme peuvent influencer cette partition. Ces paramètres sont testés à l’aide de modèles analogiques pour deux géométries de butées rigides. Les expériences montrent que la pente de stabilité du prisme reste constante et proche de αcoulomb pour desobliquités de convergence variables. L’analyse des résultats suggère que l’équation sur la stabilité des prismes de Coulomb s’appliquerait seulement dans la direction de convergence et donc que les paramètres déduits de la théorie seraient uniquement valables dans cette direction. On constate également que le taux de partition augmente avecl’obliquité de convergence et que la partition se produit quelle que soit la friction basale. La morphologie de la structure est modifiée au-delà d’une certaine valeur d’obliquité. Dès lors, un faisceau cisaillant s’initie.Nos modèles montrent la présence de structures obliques associées au décrochement et localisées à l’aplomb dela discontinuité de vitesse. Des structures semblables à celles-ci ont été observées en Nouvelle-Zélande au niveau duprisme d’Hikurang

Tectonic segmentation of the north Andean margin : impact of the Carnegie ridge collision

The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes Mu is greater than 7.8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished : region 1 - from 6°N to 2.5°N with steep ESE-dipping subduction and a narrow volcanic arc ; Region 2 - from 2.5°N to 1°S showing an intermediate-depth seismic gap and a broad volcanic arc ; Region 3 - from 1°S to 2°S with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 - south of 2°S with flat subduction and no modern volcani arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. This findings conflict with previous tectonic models suggesting that the Canergie Ridge entered the trench at 1 Ma. Futhermore, the anomalous geochemical (adakitic) signature of the volcanoes of the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly, subducting Canergie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Canergie Ridge collision in the upper plate causes trnaspressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system... (D'après résumé d'auteur

Mechanical decoupling and basal duplex formation observed in sandbox experiments with application to the Western Mediterranean Ridge accretionary complex

Sandbox experiments of accretionary wedges were performed incorporating a thin weak layer of micro glass beads. The impact of heterogeneous sedimentary input on wedge mechanics, evolution and mass transfer was investigated. We report the first experimentally documented growth of basal duplexes. These occurred for high basal friction conditions, with restricted output of the lower section. The upper and lower sections were completely decoupled due to the intervening layer of glass beads, with frontal accretion occurring in the upper section simultaneously with basal duplex formation and underplating of subsequent generations of duplexes. IMERSE multichannel seismic reflection data from the Western Mediterranean Ridge (WMR) image Tertiary clastics beneath a thick section of Messinian evaporites. The base of the evaporites is identified as the primary décollement for deformation in the frontal part of the accretionary complex. Constriction of the channel of subducting Tertiary sediments, as well as internal deformation observed as arcward-dipping reflectors argue for basal underplating and/or two different active décollements. We propose an evolution of the WMR in accordance with the sandbox experimental results. A weak mid-level detachment (base of evaporites) combined with a strong basal detachment produce mechanical decoupling and basal accretion of toeward-verging duplexes