François Jouanne

Current strain regime in the Western Alps from continuous Global Positioning System measurements, 1996–2001

Four to six years of continuous measurements at 10 permanent Global Positioning System sites in the Western Alps show horizontal residual velocities of ,2 mm/yr with respect to stable Europe; uncertainties range from 0.3 to 1.4 mm/yr. These velocities and the associated strain-rate field indicate that the central part of the range is currently dominated by east-west extension, whereas the southern part shows north-south to northwest-southeast compression. The geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the Adriatic microplate with respect to Eurasia. This rotation, together with the arcuate shape of the contact between the Adriatic microplate and the Alps, induces dextral shear kinematic boundary conditions across the Western Alps, with an additional divergence component in their central part and in Switzerland, and a convergence component in their southern part.

Oblique convergence in the Himalayas of western Nepal deduced from preliminary results of GPS measurements

A GPS network consisting of 29 sites was installed in central and western Nepal, with measurements taken in 1995 and partial remeasurements in 1997. Data suggest 15 +/−5 mm/yr of N180° convergence between the Higher Himalayas and India, a result that is consistent with N‐S shortening across the arcuate shape of the Nepalese Himalayas and an oblique underthrusting of the Indian crust below the High Himalayas of western Nepal. A 4 +/−3 mm/year E‐W extension and deviation of the principal shortening axes are inferred east of 83°E, where Quaternary faults (Darma‐Bari Gad fault system and Thakkhola graben) delineate a crustal wedge. This wedge is located on the SE projection of the Karakorum fault and may segment the Himalayan thrust belt. The convergence between the outer belt of western Nepal and India is less than 3 mm/yr, an attenuation consistent with creep on a dislocation locked beneath the Lesser Himalayas. A preliminary model suggests that this N 120°E striking dislocation is affected by a 19 mm/yr thrust component and a 7 mm/yr right lateral component.

Bridging onshore and offshore present‐day kinematics of central and eastern Mediterranean: Implications for crustal dynamics and mantle flow

We present a new kinematic and strain model of an area encompassing the Calabrian and Hellenic subduction zones, western Anatolia and the Balkans. Using Haines and Holts (1993) method, we derive continuous velocity and strain rate fields by interpolating geodetic velocities, including recent GPS data in the Balkans. Relative motion between stable Eurasia and the western Aegean Sea is gradually accommodated by distributed N-S extension from Southern Balkans to the Eastern Corinth Gulf, so that the westward propagation of the North Anatolian Fault (NAF) throughout continental Greece or Peloponnesus is not required. We thus propose that the NAF terminates in north Aegean and that N-S extension localized in the Corinth Gulf and distributed in Southern Balkans is due to the retreat of the Hellenic slab. The motion of the Hyblean plateau, Apulia Peninsula, south Adriatic Sea, Ionian Basin and Sirte plain can be minimized by a single rigid rotation around a pole located in the Sirte plain, compatible with the opening the Pelagian rifts (2–2.5 mm/yr) and seismotectonics in Libya. We interpret the trenchward ultraslow motion of the Calabrian arc (2–2.5 mm/yr) as pure collapse, the Calabrian subduction being now inactive. In the absolute plate motion reference frame, our modeled velocity field depicts two toroidal crustal patterns located at both ends of the Hellenic subduction zone, clockwise in NW Greece and counter-clockwise in western Anatolia. We suggest the NW Greece toroidal pattern is the surface expression of a slab tear and consequent toroidal asthenospheric flow.

Present‐day vertical displacements in the north‐western Alps and southern Jura Mountains: Data from leveling comparisons

Two high-precision leveling networks were successively surveyed in France, the NGF, measured during the 1886–1907 period, and the IGN69, measured from 1965 to 1979. The accuracy of these levelings (standard deviation of 1.8mm/√km to 3.8mm/√km) allows us to compute the vertical displacements of the benchmarks between two different eras. The results indicate the occurrence of discrete zones of uplift and subsidence: (1) a regional uplift (up to 1.4 mm/yr) of the Subalpine Massifs; (2) an important uplift of the internal Jura (up to 2 mm/yr); (3) a relative subsidence of the southern part of the Jura (0.8 mm/yr); and (4) a relative subsidence of the Bresse Basin with respect to the external Jura. Comparing the spatial distribution of zones of uplift and their respective vertical displacement rates with a regional structural cross section leads to the conclusion that present-day uplift of the Belledonne and Bornes Massifs and of the internal parts of the Jura Mountains, can be explained by crustal shortening along a major basement-involving thrust fault. This fault ramps up under the Bornes Massif from a depth of 12 km to 7 km, turns into a flat under the Molasse Basin, ramps up to the top of the basement at the north-eastern margin of the internal Jura Mountains, and reaches the surface in the external Jura. The Saleve ramp-anticline is carried by a bifurcation of this thrust. Horizontal displacement rates of 6 mm/yr at the Bornes ramp, 2 mm/yr at the Saleve ramp, and 4 mm/yr at the internal Jura ramp have been determined by inversion of profiles of uplift rates. Whether this basement-involving thrust fault was already active during the Miocene main folding phase of the Jura Mountains or whether it was activated only during Pliocene-Pleistocene times is subject to debate.

A two‐magma chamber model as a source of deformation at Grímsvötn Volcano, Iceland

Grimsvotn Volcano is the most active volcano in Iceland, and its last three eruptions were in 1998, 2004, and 2011. Here we analyze the displacement around Grimsvotn during these last three eruptive cycles using 10 GPS stations. The observed displacements in this region generally contain a linear component of tectonic and glacio-isostatic origin, in agreement with the previously estimated values of plate motions and vertical rebound. Larger amplitude deformation observed close to Grimsvotn at the GFUM continuous GPS station clearly reflects a major volcanic contribution superimposed on a tectonic component. We estimate and subtract the tectonic trend at this station using regional observed displacement. The direction and pattern of the residual volcanic displacement (for coeruptive and intereruptive periods) are consistent for all three of these eruptive cycles. The posteruptive inflation is characterized by an exponential trend, followed by a linear trend. In this study, we explain this temporal behavior using a new analytic model that has two connected magma chambers surrounded by an elastic medium and fed by a constant basal magma inflow. During the early posteruptive phase, pressure readjustment occurs between the two reservoirs, with replenishment of the shallow chamber from the deep chamber. Afterward, due to the constant inflow of magma into the deep reservoir, the pressurization of the system produces linear uplift. A large deep reservoir favors magma storage rather than surface emission. Based on displacement measured at GFUM station, we estimate an upper limit for the radius of the deep reservoir of ∼10 km.

ESTIMATING PRESENT-DAY DISPLACEMENT FIELDS AND TECTONIC DEFORMATION IN ACTIVE MOUNTAIN BELTS : AN EXAMPLE FROM THE CHARTREUSE MASSIF AND THE SOUTHERN JURA MOUNTAINS, WESTERN ALPS

Abstract Determination of relative movements between the alpine foreland and the External Crystalline Massif is a key-point for the understanding of the present-day tectonics of the western Alps. In this study we try to test the continuity of the present-day tectonics with the Mio-Pliocene deformation. In particular, we will test if the present-day displacements are localized along the thrusts of the Jura Mountains, or along a blind thrust in the Bas Dauphine Molasse Basin. Definition of relative movements is achieved by several methods, including a comparison of two high precision leveling networks to estimate vertical displacements, horizontal deformation measurements performed by triangulation/triangulation and triangulation/GPS comparison, in situ stress measurements performed in the different tectonic units and geomorphologic observations that constrain the location and the magnitude of the Quaternary deformation. Comparison of leveling data demonstrates: (1) an uplift of the southern Bas Dauphine Molasse Basin relative to its northern part (0.8 mm/year), also revealed by geomorphologic analysis, (2) a significant uplift of the most external jurassian anticlines (0.8 to 2 mm/year), also recorded by the deformation of a paleo-river bed, and (3) an important uplift (up to 2 mm/year) of the Subalpine Massifs. The horizontal strain estimated from comparison of horizontal geodetic data (triangulation, GPS) shows (1) a NW–SE directed shortening between the eastern Chartreuse Massif and the Bas Dauphine Molasses Basin (approximately 3 mm/year), (2) an E–W-directed shortening in the Jura Mountains (approximately 4–3 mm/year) and (3) a dextral strike-slip motion consistent with focal mechanisms along a NNE–SSW direction between the eastern Chartreuse Massif and the eastern Belledonne Massif. These data reveal a present-day strain partitioning between the Belledonne External Crystalline Massif and the Bas Dauphine Molasses Basin. The westward motion of the Subalpine Massifs is partitioned along two southern jurassian thrust-folds, and a dextral NNE–SSW strike-slip shear zone between the Chartreuse Massif and the Belledonne Massif. This strain partitioning is also accompanied by a stress partitioning between the alpine foreland and the External Crystalline Massifs.

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.

Present‐day deformation of northern Pakistan from Salt Ranges to Karakorum Ranges

Episodic GPS measurements are used to quantify the present-day velocity field in the northwestern Himalaya from the southern Pamir to the Himalayan foreland. We report large postseismic displacements following the 2005 Kashmir earthquake and several mm/yr thrusting of the central segment of the Salt Ranges and Potwar Plateau over the foreland, westward thrusting of Nanga Parbat above the Kohistan Plateau, and ~12 mm/yr SSE velocities of the Karakorum Ranges and of the Deosai and Kohistan Plateaus relative to the Indian Plate. Numerical simulations allow to determine a first approximation of slip along active faults: (1) substantial creep of ~87 mm/yr between 2006 and 2012 along the flat northeast of the Balakot-Bagh Thrust affected by the 2005 earthquake; (2) ~5 mm/yr slip of the central segment of the Salt Ranges and Potwar Plateau, whereas their western boundaries are clearly inactive over the time span covered by our measurements; (3) 13 mm/yr ductile slip along the Main Himalayan Thrust modeled by a dislocation dipping 7° northward, locked at a depth of 15 km; and (4) ~20 mm/yr slip along the shear zone forming the western boundary of Nanga Parbat, between depths of 1.6 and 6.5 km. Residuals velocities suggest the existence of left-lateral strike slip along the Jhelum Fault.

Long period interferograms reveal 1992–1998 steady rate of deformation at Krafla Volcano (North Iceland).

We formed interferograms of ERS-SAR scenes covering the area of Krafla (N. Iceland) with time span values of up to six years (1992–1998). Our data reveals a steady deformation rate at Krafla and within its fissure swarm, with values reaching +2.1 cm/y in the ground to satellite direction, at the volcano. The area affected by deformation extends 20 km both north and south of the volcano. The best fit dislocation model consists of sills, to the north and south of the volcano, and a magma chamber, located below the volcano, all of them undergoing contraction.

Geodetic exploration of strain along the El Pilar Fault in northeastern Venezuela

We use Global Navigation Satellite Systems observations in northeastern Venezuela to constrain the El Pilar Fault (EPF) kinematics and to explore the effects of the variable elastic properties of the surrounding medium and of the fault geometry on inferred slip rates and locking depth. The velocity field exhibits an asymmetric velocity gradient on either side of the EPF. We use five different approaches to explore possible models to explain this asymmetry. First, we infer a 1.6 km locking depth using a classic elastic half-space dislocation model. Second, we infer a 1.5 km locking depth and a 0.33 asymmetry coefficient using a heterogeneous asymmetric model, including contrasting material properties on either side of a vertical fault, suggesting that the igneous-metamorphic terranes on the northern side are ~2 times more rigid than the sedimentary southern side. Third, we use a three-dimensional elastostatic model to evaluate the presence of a compliant zone, suggesting a 30% reduction of rigidity in the upper 3 km at the depth of a 1 to 5 km wide fault zone. Fourth, we evaluate the distribution of fault slip, revealing a widespread partial creep pattern in the eastern upper segment, while the upper western segment exhibits a partially locked area, which coincides with the rupture surface of the 1797 and 1929 earthquakes. To supplement these models, we upgrade the previously published displacement simulation method using nonvertical dislocations with data acquired between 2003 and 2013. The localized aseismic displacement pattern associated with creeping or partially creeping fault segments could explain the low level of historic seismicity.