Frederic Masson

SEISMIC TOMOGRAPHY OF NORTHERN TIBET AND KUNLUN : EVIDENCE FOR CRUSTAL BLOCKS AND MANTLE VELOCITY CONTRASTS

Although the crust and mantle of the Tibet Plateau reveal vital information for understanding the interplay of dynamic processes that has governed its recent uplift and growth, their deep, physical and thermal structure remains poorly understood. In order to throw light on the structure and, hence, to constrain the processes and models tied to them, we performed a teleseismic experiment on a 600 km long profile across the northern part of the plateau, the Kunlun range and the Qaidam basin. The 400 km deep tomographic image we obtain has a resolution < 50 km, over one order of magnitude better than achieved in previous, broader-scale studies. At relatively shallow depth (< 100 km), the tomographic cross-section clearly reveals that the mid-lower crust and upper lithosphere of northern Tibet is an assemblage of blocks with different velocities and thicknesses, hence different natures, histories and ages. The crust of the Qiantang block appears to be thickest (≈ 70 km), with the lowest velocity. Along the north edge of that block, the Jinsha suture, although not remarkable in the local geology and topography, stands as a particularly sharp and prominent crustal boundary. The crust and lithosphere of the region between that suture and the Qaidam (Bayan Har-Songpan, South Kunlun) appear to be thinnest (≈ 50 km) and with the highest velocity. At greater depth (> 150 km), our experiment confirms the existence of a bulky low-velocity zone in the mantle beneath the northernmost reaches of the plateau. The tomogram helps assess with unprecedented accuracy the location and shape of this low-velocity anomaly, which forms a 250–300 km wide dome rising to about 150–200 km, south of the Kunlun range, roughly coincident with the region where the strongest shear-wave splitting, hence horizontal anisotropy, has recently been found. That dome lies between two relatively high velocity zones with opposite dips, one towards the north, under the Qaidam basin, the other towards the south, beneath the Quangtang platform. The implications that this new, detailed image of the deep structure of northern Tibet has upon the uplift and thermo-mechanical evolution of the plateau are discussed.

Seismic tomography of continental rifts revisited: from relative to absolute heterogeneities

Abstract Tomographic images for four major continental rift zones, namely the southern Rhine Graben (SRG, Germany/France), the Gregory rift (Kenya) which is the central part of the East African rift system, the Rio Grande rift (RGR) in the United States and the Lake Baikal rift zone (LBR) in Russia have been revisited by calculating and comparing absolute velocity models. The four rifts exhibit strong structural differences in the uppermost mantle down to more than 300-km depth, suggesting major differences in their geodynamic evolution albeit their similarity in age and similar surface expression. The comparative analysis suggests that tomographic images of rift zones can be used to characterize continental rifts, once the corrections to obtain absolute velocities have been carried out. Our results suggest that while the Kenya and the Rio Grande rift may be considered active with large upwelling plumes being the main controlling factor in the evolution, the southern Rhine Graben and the Lake Baikal rift are more likely passive rifts, where complex regional stress fields and inherited structures play the governing role in the evolution.

First global positioning system results in northern Myanmar: Constant and localized slip rate along the Sagaing fault

We present the first recorded global positioning system (GPS) data from Myanmar measured at the northern tip of the Sagaing fault. This area is in a very complex geodynamic context, where rigid and semirigid plates interact. The 12 GPS sites measured in 2005 and 2008 in northern Myanmar show that the slip rate is 18 mm/yr and is localized along a single active narrow fault trace (<20 km wide). The same rate was previously demonstrated and remeasured, 500 km southward, in central Myanmar. Despite the geodynamic regional complexity induced by interaction between the Sunda and India plates, the Burma microplate, and the highly deformable eastern Himalayan syntaxis, the slip rate remains surprisingly constant along this fault. However, the modeled locking depth varies from 20 km in central Myanmar to 5 km in the north.

On the use of GPS tomography to investigate water vapor variability during a Mistral/sea breeze event in southeastern France

Global Positioning System (GPS) tomography analyses of water vapor, complemented by high-resolution numerical simulations are used to investigate a Mistral/sea breeze event in the region of Marseille, France, during the ESCOMPTE experiment. This is the first time GPS tomography has been used to validate the three-dimensional water vapor concentration from numerical simulation, and to analyze a small-scale meteorological event. The high spatial and temporal resolution of GPS analyses provides a unique insight into the evolution of the vertical and horizontal distribution of water vapor during the Mistral/sea-breeze transition.

Composition of the crust and upper-mantle in the Central Andes (19°30′S) inferred from P wave velocity and Poisson's ratio

The present-day crustal structure of the Andean chain results from deformation processes and magmatic activity since the onset of subduction. One line of investigation to resolve questions about the mechanism of crustal thickening is to get information about the composition of the crust and upper-mantle by imaging the structures in velocity and Poissons ratio (or Vp/Vs). A high density seismic network was operated during six months in 1994 across the whole Andean chain. The P and S travel times of the best located local earthquakes, mainly situated in the subduction zone, have been inverted for Vp and Vp/Vs using Thurbers 3D iterative simultaneous inversion method. The resulting models, giving short-scale information about the crustal and upper-mantle composition, provide new limits on the respective involvement of crustal shortening and magmatism in the thickening of the Andean crust. Beneath the Western Cordillera, the seismic properties are well explained by typical active volcanic arc processes, and the thickening of the crust is explained by magmatic addition. Conversely, beneath the Central Altiplano Basin and the Eastern Cordillera, our results preclude significant volume of magmatic addition from the mantle and reinforce crustal shortening as the main thickening process. Moreover, cratonic lower crust and subcontinental lithospheric upper-mantle are observed under the Eastern Cordillera up to 67°W, evidence of the underthrusting of the Brazilian craton, which drives the crustal shortening. Beneath the Central Altiplano Basin, a thin lithospheric mantle is observed above a well marked asthenospheric wedge which extends in depth below the western part of the Eastern Cordillera. Thus partial melt is observed in the shallow mantle west of the Western Cordillera up to the Brazilian craton.

Present-day uplift of the western Alps

Collisional mountain belts grow as a consequence of continental plate convergence and eventually disappear under the combined effects of gravitational collapse and erosion. Using a decade of GPS data, we show that the western Alps are currently characterized by zero horizontal velocity boundary conditions, offering the opportunity to investigate orogen evolution at the time of cessation of plate convergence. We find no significant horizontal motion within the belt, but GPS and levelling measurements independently show a regional pattern of uplift reaching ~2.5 mm/yr in the northwestern Alps. Unless a low viscosity crustal root under the northwestern Alps locally enhances the vertical response to surface unloading, the summed effects of isostatic responses to erosion and glaciation explain at most 60% of the observed uplift rates. Rock-uplift rates corrected from transient glacial isostatic adjustment contributions likely exceed erosion rates in the northwestern Alps. In the absence of active convergence, the observed surface uplift must result from deep-seated processes.

Continuous Monitoring and Near-Real Time Processing of GPS Observations for Landslide Analysis: A Methodological Framework

The objective of this work is to present a methodology for the near-real time characterization of displacements using permanent GPS stations on landslides. In France, several GPS receivers have been installed on active landslides since a few years. These landslides show very different displacement rates and kinematic regimes. For the monitoring of landslides where the required degree of accuracy is millimetric, GPS has been mainly used for repeated measurements, as a complement to conventional geodetic methods. Permanent monitoring is still not usually performed operationally.

Aseismic slip and seismogenic coupling in the Marmara Sea: What can we learn from onland geodesy?

Ever since the Mw7.4 Izmit earthquake in 1999, evaluation of seismic hazard associated with the last unbroken segments of the North Anatolian fault is capital. A strong controversy remains over whether Marmara fault segments are locked or are releasing strain aseismically. Using a Bayesian approach, we propose a preliminary probabilistic interseismic model constrained by published GPS data sets. The posterior mean model show that Ganos and Cinarcik segments are locked while creep is detected in the central portion of Marmara fault. Our analysis, however, reveals that creeping segments are associated with large model uncertainties, which mainly results from the sparsity of current geodetic observations. We then discuss how the GPS network can be improved to attain more reliable assessment of interseismic slip rates. With this purpose, we implement a network optimization procedure to identify the most favorable distribution of stations measuring strain accumulation in the Marmara Sea.

GURN (GNSS Upper Rhine Graben Network): Research Goals and First Results of a Transnational Geo-scientific Network

The Upper Rhine Graben (URG) is a north-northeast trending rift system belonging to the European Cenozoic Rift System. Today, the southern part of the URG is seismically still active. Earthquakes of magnitude five have a recurrence time of approximately a few decades. In order to monitor and to determine recent crustal displacements in the URG area, the transnational cooperation GURN (GNSS Upper Rhine Graben Network) was established in September 2008. Within GURN geo-scientific research is carried out. The focus is on processing and analysing of observation data of continuously operating GNSS (Global Navigation Satellite Systems, e.g. GPS) sites.

La campagne Cops : genèse et cycle de vie de la convection en région montagneuse

The Convective and Orographically- induced Precipitation Study (COPS) is a coordinated international project, comprised of an observational field campaign and a research programme aiming to advance the quality of fore- casts of orographically-induced pre- cipitation by four-dimensional obser- vations and modelling of its life cycle. The COPS field campaign took place during June-July-August 2007 over eastern France and south-western Germany. Its main objective was to provide an unprecedented comprehen- sive set of in situ and remotely-sensed meteorological observations of the entire depth of the troposphere. After a short overview of the project, the paper focuses on the French contribution to the COPS field phase, describes the experimental setup and highlights some key observations.