Éditeur(s) : HAL CCSD
Résumé : In SW Yukon – SE Alaska, the boundary between the Pacific and North America plates is characterized by a syntaxis at the transition between the Aleutian subduction to the W and the Fairweather – Queen Charlotte strike-slip faults to the SE. The relative motion is oblique to the main fault structures, and the area is marked by the Yakutat block collision. From the Chugach – Saint Elias mountains in the plate boundary zone (up to 6 000 m high) to the intraplate strike-slip faults, markers of the present-day deformation give information on its partition in the system.During my PhD, I first measure surface deformation using a dense GPS network, deployed up to 500 km inland the North America plate. After precise processing and corrections of transient effects in the area (postseismic and glacial isostatic rebound), a new residual velocity field is produced for the syntaxis area, from which I derive strain rates. Those data allow me to quantify the fault slip rates for Fairweather and southern Denali strike-slip faults, and to characterize a bi-modal deformation pattern: Along the plate boundary, the deformation is localized on large-scale structures (accretionary prism to the W, Fairweather to the E); In the syntaxis area, strain rates are the highest and the GPS data shows a diffuse intraplate deformation, similar to an indentor pattern. The Yakutat block seems to strongly drive the North America plate deformation.This indentor pattern induces strong lateral variations on the large intraplate faults: the Denali – Totschunda – Duke River system. In a second part, I realize a regional geomorphological study to characterize the role and slip rate of those faults. From very high-resolution Digital Elevation Models (~ 1 m), a detailed cartography is done. On the basis of fieldwork observations, I measure offsets of fluvial and glacial markers, which are sampled for dating. A dextral cumulative deformation is highlighted on the northern Denali Fault, where as all southern Denali is marked by vertical deformation. This study allows me to quantify new slip rates for the system Denali – Totschunda – Duke River, and to show the leading role of the Totschunda (~ 14 mm/a) and Duke River (~ 6 mm/a) faults, contrary to the Denali Fault (~ 1 mm/a) North of the syntaxis.The new tectonic model for the Yakutat collision provides an important case study for the understanding of indentor systems. The concomitance of a rigid-block deformation (to the West) and diffuse deformation (to the East), as well as the near-zero slip rate on the lithospheric-scale southern Denali Fault highlight the major control of boundary conditions and the structural heritage on the orogen deformation.
Au SO Yukon – SE Alaska, la frontière entre les plaques Pacifique et Nord-Amérique est marquée par une syntaxe à la transition entre la subduction des Aléoutiennes à l’O et le décrochement de Fairweather – Queen Charlotte au SE. Le mouvement relatif est oblique et la région est marquée par la collision du bloc Yakutat. De la chaine des Chugach – Saint Elias en frontière de plaque (jusqu’à 6 000 m d’altitude) aux grands décrochements intraplaque, les marqueurs de la déformation actuelle apportent des informations sur sa partition dans ce système.Au cours de cette thèse, je m’intéresse dans un premier temps à mesurer la déformation de surface à l’aide d’un réseau GPS dense, déployé jusqu’à 500 km à l’intérieur de la plaque Nord-Amérique. Après un travail minutieux de traitement des données et de correction des effets transitoires (rebond post-glaciaire et post-sismique), un champ de vitesses résiduelles robuste et inédit est produit au niveau de la syntaxe, dont je dérive des taux de déformation. Ces données me permettent de quantifier les vitesses de glissement les failles décrochantes de Fairweather et Denali au Sud, et également de caractériser une déformation bimodale : En frontière de plaques, la déformation est localisée sur les grandes structures (prisme d’accrétion à l’O, Fairweather à l’E) ; Au niveau de la syntaxe, les taux de déformation sont les plus importants et les données GPS mettent en évidence une déformation intraplaque diffuse, similaire à un champ attendu à l’aplomb d’un indenteur. Le bloc Yakutat semble donc contrôler fortement la déformation de la plaque Nord-Amérique.Ce champ de déformation induit des variations latérales fortes sur les grandes failles intraplaque : le système Denali-Totschunda-Duke River. Dans une seconde partie, je réalise une étude géomorphologique régionale pour caractériser le rôle et la vitesse de ces failles. À partir de Modèles Numériques de Terrain très haute résolution (~ 1 m), une cartographie de détail est réalisée. Puis une mission de terrain me permet de mesurer des décalages sur des marqueurs fluviatiles et glaciaires et collecter des échantillons pour les dater. Je mets en évidence une déformation cumulée dextre sur le segment Nord de la faille de Denali, alors que toute la partie Sud déforme verticalement la surface. Cette étude me permet de quantifier de nouvelles vitesses de failles sur le système Denali-Totschunda-Duke River, et de montrer le rôle prépondérant des failles de Totschunda (~ 14 mm/a) et Duke River (~ 6 mm/a) contrairement à la faille de Denali (~ 1 mm/a) au Nord de la syntaxe.Le nouveau modèle cinématique proposé pour la région de collision Yakutat permet d’apporter un nouvel exemple à la compréhension des systèmes d’indenteur. La concomitance d’une déformation de blocs (à l’Ouest) et diffuse (à l’Est), ainsi que l’absence de déformation sur la faille lithosphérique de Denali Sud met en évidence le contrôle majeur des conditions aux limites et de l’héritage structural dans la déformation des orogènes.
https://hal.archives-ouvertes.fr/tel-01241688

tel-01241688
https://hal.archives-ouvertes.fr/tel-01241688
https://hal.archives-ouvertes.fr/tel-01241688/document
https://hal.archives-ouvertes.fr/tel-01241688/file/manuscrit_anais_v2_optimise.pdf

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Estimating how topography is maintained provides insights into the different factors responsible for surface deformations and their relative roles. Here, we develop a new and simple approach to assess the degree of isostatic compensation of continental topography at regional scale from GOCE gravity gradients. We calculate the ratio between the radial gradient observed by GOCE and that calculated from topography only. From analytical and statistical formulations, simple relationships between this ratio and the degree of compensation are obtained under the Airy–Heiskanen isostasy hypothesis. Then, a value of degree of compensation at each point of study area can be easily deduced. We apply our method to the Alaska-Canada Cordillera and validate our results by comparison with a standard isostatic gravity anomaly model and additional geophysical information for this area. Both our GOCE-based results and the isostatic anomaly show that Airy–Heiskanen isostasy prevails for the Yukon Plateau whereas additional mechanisms are required to support topography below the Northwest Territories Craton and the Yakutat collision zone.
ISSN: 0956-540X

hal-01467540
https://hal.archives-ouvertes.fr/hal-01467540
https://hal.archives-ouvertes.fr/hal-01467540/document
https://hal.archives-ouvertes.fr/hal-01467540/file/cadioGJI2016.pdf
DOI : 10.1093/gji/ggw281

Éditeur(s) : HAL CCSD Seismological Society of America
Résumé : International audience
The objective of this paper is to provide a better understanding of the relationship between the microseismicity, active tectonics, and crustal structures in the southwest Yukon Territory, Canada, in order to improve seismic‐hazard assessments in this region. We utilize data from a new dense seismic array that was deployed in the southwest Yukon in the summer of 2010. Data from the new seismic array significantly improve the magnitude completeness level in the region from ML 3.0 to 1.0. We analyze 980 events ranging in magnitudes from ML 0.2 to 4.7, at depths from 0 to 35 km. Relocation analysis using the progressive multiple event location shows that seismicity is concentrated in four main areas: (1) Yakutat block northern boundary-Fairweather fault, (2) Duke River fault, (3) southern Denali fault, and (4) a previously unrecognized northeast trending cluster that may highlight a previously unknown active fault. This cluster may contribute to stress and strain transfer inland from the Yakutat block region.
ISSN: 0037-1106

hal-00940935
https://hal.archives-ouvertes.fr/hal-00940935
DOI : 10.1785/0120130068

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The Yakutat-St. Elias collision in SE Alaska and adjacent Canada represents a prime example of present-day tectonics associated with an indentor corner. Its eastern syntaxis is marked by high exhumation, a sharp structural bend, and strain concentration at the transition from shortening to oblique transpression. Here we present GPS velocity and strain rate fields that cover the syntaxis, including 11 new stations in the core of the St. Elias Mountains. These data are corrected for transient deformation (glacial isostatic adjustment and postseismic and interseismic loading) to produce residual velocities and strain rates representative of long-term tectonics. The main features of these velocity and strain rate fields are a peak in strain rates (strain knot) in the syntaxis at the junction between the main fault systems and a rapid rotation from convergence-parallel to convergence-normal orientations of the velocities and shortening axes around the syntaxis, leading to shortening across the southern Denali Fault. These features are consistent with the strain and tectonic patterns expected near an indentor corner at the transition from shortening to transpression, with a combination of diffuse and localized deformation. The GPS velocities and strain rates show diffuse deformation around the syntaxis, from pure convergence-parallel shortening in the orogenic wedge to oblique extension that accommodates the strain rotation at the front of the syntaxis. This indentor-corner model also results in a near-zero strike-slip rate on the southern Denali Fault and shows no clear evidence for a throughgoing fault hypothesized to link the Fairweather and Totschunda Faults.
ISSN: 0148-0227

hal-01243510
https://hal.archives-ouvertes.fr/hal-01243510
DOI : 10.1002/2014JB011842