Thierry Villemin

Effective tension-shear relationships in extensional fissure swarms, axial rift zone of northeastern Iceland

Abstract The geometry of fracture systems in selected areas of the active Krafla fissure swarm, mid-Atlantic ridge, northeastern Iceland, is analysed. Based on geodetic analysis of the present-day topography at the top of Holocene basaltic lava flows which fill the axial rift zone, the deformation of this initially horizontal surface can be reconstructed. Extensional deformation is localised at all scales and block tilting, though present, remains minor. Using simple models of the surface expression of normal faults, the geometrical characteristics of the topographic features related to active deformation during tectonic-volcanic events are quantitatively analysed. At crustal depths of about 1 km, normal faults are present and have an average 70 ° dip. Comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone, at palaeodepths of 1–2 km, indicates that this dip determination is valid. Comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin, newly formed brittle crust of the rift zone. In the axial oceanic rift zone of NE Iceland, the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense, their relative importance depending on depth. Absolute tension dominates in the uppermost several hundred metres of the crust, resulting in the development of fissure swarms. Effective tension plays an important role at a deeper level (2–5 km), because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections. At crustal depths of about 1 km, normal shear prevails along fault planes which dip 60 °–75 °. This importance of normal shear at moderate depth, between upper and lower crustal levels where tension prevails, is pointed out. Within the extensional context of rifting, these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure.

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.

Brittle deformation and fracture patterns on oceanic rift shoulders: the esja peninsula, SW Iceland

Abstract This paper provides detailed structural data on the crustal deformation of the Esja peninsula, at the southwestern margin of the rift zone in SW Iceland. Forty percent of the faults are strike-slip with steep dips, 35% are normal dip-slip with dips ranging from 50° to 90°. For normal faults, E-W, NE-SW and NNE-SSW are the three major trends. The paleostress tensors determined using the fault data, indicate three main states of stress. Two extensional regimes (σ3 N180°E ±10° and N110°E ±10°) are related to normal slip faults. The third is a compressive stress regime with σ1 trending N10°E-N30°E. Associations between strike-slip and dip-slip faults are often observed, resulting from both the geometrical requirements of interactions between faulted blocks and from local-regional permutations between σ1 and σ2. We note a constant obliquity of 10–20° between the trend perpendicular to the regional structures and the computed direction of extension. This conclusion is also supported by the major trends of dykes and tensional fractures. There are two mechanisms of volcanic injection (local sheets and regional dykes) resulting in power-law cumulative thickness distributions with exponents of 1.5 and 3.0, respectively.

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.

Microfracture pattern compared to core-scale fractures in the borehole of Soultz-sous-Forêts granite, Rhine graben, France

Microfractures appearing in thin section as fluid inclusion trails in quartz crystals were studied in four core samples of Soultz-sous-Forets granite. Their orientations in four series of three mutually perpendicular thin sections were estimated using a previously described apparent dip method and a new method involving measurements of strike and apparent dips. Three samples display three microfracture sets and one sample displays two sets. In all samples, one set is nearly vertical and strikes N–S. In two samples, one and two sets are nearly vertical and strike E–W. In two samples, two sets strike NW–SE: one is vertical, the other dips gently to the NE (or SW). Comparing microfracture and mesofractures sets in the same cores shows that (1) the N–S microfracture set is always dominant at both scales and (2) all other microfracture sets have no mesoscopic counterpart. The N–S microfracture sets could have been created during E–W extension of earliest Cenozoic age (Rhine Graben rifting). Differences between the two scales are explained by a σ1/σ2 switching which occurred at the crystal scale and generated mutually perpendicular cracks.

Seismic risk at the rift‐transform junction in North Iceland

The current tectonics of north Iceland are characterised by max M 6–7 earthquakes in the transform zone between the northern rift zone and the Kobleinsey Ridge. The last rifting period (1975–1984) was associated with an opening of up to 9 m along the Krafla fissure swarm. This period was followed by the disappearance of microseismicity along the eastern part of Husavik-Flatey Fault, the main structure of the transform zone. A GPS network was occupied in 1995 and again in 1997 to study present-day deformation on both sides of the on-land part of HFF and around its junction with the northern rift zone. The main deformational features observed are: (1) in Tjornes, the HFF is locked above the ductile/brittle boundary below which ductile creep is occurring, and (2) aseismic opening along the Theystareykir and Krafla fissure swarms and along a N 120°E superficial fault that marks the southern boundary of Tjornes.

Déformation verticale actuelle dans la partie sud du fossé d'Alsace (France)

Abstract Active deformation in the southern part of the Rhinegraben, studied by comparing leveling measurements, is characterized by vertical movements. Upward movements greater than 1 mm·y −1 have been measured near Mulhouse and Selestat. Smaller rates of uplift averaging 1 mm·y −1 were also observed above some groups of salt plugs striking NW-SE. Both types of vertical uplift are compatible with the regional stress field. The movements may be associated with the reactivation of NE-striking Variscan thrusts and with continuing upward growth of those salt plugs that are perpendicular to the minimum horizontal stress.

OBSIFRAC: database-supported software for 3D modeling of rock mass fragmentation

Under stress, fractures in rock masses tend to form fully connected networks. The mass can thus be thought of as a 3D series of blocks produced by fragmentation processes. A numerical model has been developed that uses a relational database to describe such a mass. The model, which assumes the fractures to be plane, allows data from natural networks to test theories concerning fragmentation processes. In the model, blocks are bordered by faces that are composed of edges and vertices. A fracture can originate from a seed point, its orientation being controlled by the stress field specified by an orientation matrix. Alternatively, it can be generated from a discrete set of given orientations and positions. Both kinds of fracture can occur together in a model. From an original simple block, a given fracture produces two simple polyhedral blocks, and the original block becomes compound. Compound and simple blocks created throughout fragmentation are stored in the database. Several fragmentation processes have been studied. In one scenario, a constant proportion of blocks is fragmented at each step of the process. The resulting distribution appears to be fractal, although seed points are random in each fragmented block. In a second scenario, division affects only one random block at each stage of the process, and gives a Weibull volume distribution law. This software can be used for a large number of other applications.

AN ANALYSIS OF THE NONLINEAR MAGMA‐EDIFICE COUPLING AT GRIMSVÖTN VOLCANO (ICELAND)

Continuous monitoring of seismicity and surface displacement of active volcanoes can reveal important features of the eruptive cycle. Here, high-quality GPS and earthquake data recorded at Grimsvotn volcano by the Icelandic Meteorological Office during the 2004-2011 inter-eruptive period are analyzed. These showed a characteristic pattern, with an initial ∼2-year-long exponential decay followed by ∼3-year-long constant surface-displacement inflation rate. We model it by using a one magma-reservoir model in an elastic damaging edifice, with incompressible magma and constant pressure at the base of the magma conduit. Seismicity rate and damage were first modeled, and simple analytical expressions were derived for the magma reservoir overpressure and surface displacement as functions of time. Very good fits of the seismicity and surface displacement data were obtained by fitting only three phenomenological parameters. Characteristic time and power strain show maxima from which reference times were inferred that split the inter-eruptive period into five periods. After the pressurization periods, damage occurring in the third period induced weakly nonlinear variations in magma overpressure and flow, and surface displacement. During the fourth period, the damage dominated and variations became more strongly nonlinear, the reservoir overpressure decreased and magma flow increased. This process lasted until the power strain reached its second maximum, where instability was generalized. This maximum is a physical limit, the occurrence of which shortly precedes rupture, and eventually eruption. This analysis allows characterization of the state of the volcanic edifice during the inter-eruptive period, and supports medium-term prediction of rupture and eruption.

Deformation at the northern end of the Icelandic rift mapped by InSAR (1992-2000), a decade after the Krafla Rifting Episode

From 1975 to 1984 the northern Icelandic rift underwent a rifting crisis. Large widening, rapid and local subsiding and uplifting occurred on the Krafla fracture swarm with several fissure eruptions. Since 1992 the area has been regularly covered by the two ERS radar satellites, allowing us to form 110 INSAR interferograms with 22 SAR scenes for mapping crustal deformation. At the regional scale, the signal is characterised by a series of parallel fringes oriented N15°W. The polarity of these fringes is constant throughout the mapped area. The measurement represents an increase of 1.7 cm/y in ground to satellite distance for the Western bloc. This signal is estimated to correspond to the combination of a horizontal opening movement and a vertical movement. In addition, the local signal at Krafla is marked by a series of concentric circular fringes forming a circle centred on the volcano and two U shaped structures, respectively upright to the South and upside down to the North of the volcano. The signal geometry indicates that the main component of movement affecting Krafla is vertical. The measured signal corresponds to a 2.2 cm/y increase in the ground to satellite distance, representing a maximum 2.4 cm/y ground subsidence. The large number of interferograms on which the deformation signal affecting Krafla is visible allows us to ascertain that the deformation rate remains constant since 1992.