Olivier Dauteuil

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.

Geothermal control on flow patterns in the Last Glacial Maximum ice sheet of Iceland

Because it is located both on the Mid-Atlantic Ridge and on a mantle plume, Iceland is a region of intense tectonics and volcanism. During the last glaciation, the island was covered by an ice sheet approximately 1000 m thick. A reconstruction of the ice flow lines, based on glacial directional features, shows that the ice sheet was partly drained through fast-flowing streams. Fast flow of the ice streams has been recorded in megascale lineations and flutes visible on the currently deglaciated bedrock, and is confirmed by simple mass balance considerations. Locations of the major drainage routes correlate with locations of geothermal anomalies, suggesting that ice stream activity was favoured by lubrication of the bed by meltwater produced in regions of high geothermal heat flux. Similar control of ice flow by geothermal activity is expected in ice sheets currently covering tectonically and volcanically active area such as the West Antarctic ice sheet. Copyright

Pleistocene subglacial volcanism in Iceland: tectonic implications

Abstract At several stages during the last 700 kyr, tectonic and volcanic activity due to lithospheric spreading occurred beneath a 1000–1500 m thick ice cap in Iceland. Magmatic activity has been recorded by the emplacement of subglacial volcanic edifices. Table volcanoes are the subglacial equivalent of aerial shield volcanoes. Hyaloclastite ridges are the subglacial equivalent of aerial eruptive fissures. Some hyaloclastite ridges are located in currently inactive areas, whereas they are nearly absent in some parts of the currently active Neovolcanic Zone. A part of this discrepancy can be attributed to glacial erosion. A manual reconstruction of the flowing pattern of the ice cap, based on glacial landforms, shows that some parts of the Neovolcanic Zone were occupied by fast flowing ice streams. In these areas, most hyaloclastite ridges have been removed as eruptions proceeded: fast ice flow and water/debris flows triggered by volcanic eruptions have transported subglacial volcanic products to the sea. Subglacial volcanic products have been preserved beneath ice divides, where ice motion was slower, and in some table volcanoes, where magma supply was sufficient to counteract removal by ice flow. Once the effect of glacial removal has been subtracted, the arrangement of the subglacial volcanic edifices appears clearly. Similarly to the post-glacial eruptive fissures, the hyaloclastite ridges are gathered in swarms associated with central volcanoes located in the Neovolcanic Zone. However, the area covered by hyaloclastite ridges is wider than the extent of the currently active fissure swarms. This discrepancy suggests either continuous wandering of the volcanic activity from one fissure swarm to another for the last 700 kyr, or narrowing of the active rift zone at the end of the last glaciation.

Deformation partitioning in a slow spreading ridge undergoing oblique extension: Mohns Ridge, Norwegian Sea

Although oceanic spreading is often perpendicular to the ridge trends, in some cases the angle between these two directions can be significantly less than 90° (40°–50°). This occurs because of either a bend of the ridge trend or a change of the spreading direction. We here describe oblique spreading in the Mohns Ridge, resulting in deformation partitioning between the valley walls, which are dominantly affected by strike-slip displacements, and the axial valley which is subject to nearly pure extension. The axial valley walls are characterized by en echelon normal faults affecting the walls, while the axial valley is affected by parallel faults grouped into oblique sets. These fault sets define different structures, horst or tilted blocks, that are regularly spaced inside the axial valley. Moreover some ridge segments mainly undergo pure extension, whereas others are affected by oblique extension. We explain this faulting pattern, including the along-strike and transverse variations, as a consequence of depth variations of the brittle-ductile transition.

Extensional faulting and caldera collapse in the axial region of fast spreading ridges : Analog modeling

The axial high of the East Pacific Rise (EPR) is bounded by ridge-parallel lateral grabens that develop 2-8 km off-axis. These troughs appear to lengthen away from the ridge crest, suggesting that tectonics is active at least 10 km away from the axis. Along 15-20% of the length of the ridge the axial high is notched by a summit trough 500 to 1800 m wide. These large axial troughs represent elongated collapse calderas that form when the EPR magma reservoir (comprising the melt lens and the underlying crystal mush zone) deforms and compacts during periods of waning magma supply under continuous stretching. We report the results of analog experiments performed in order to constrain the tectonic-magmatic evolution of the crestal region of fast spreading ridges and more particularly the possible link that may exist between the development of axial caldera and the creation of lateral grabens along the crestal region. We used inflatable elongated balloons filled with water as an analog of the magma reservoir. The balloon is capped with a silicone layer representing hot rocks below the brittle-ductile transition and is covered by a sand layer representing the brittle crust. The sand surface was given a dome shape that approximates the morphology of a fast spreading ridge. Mobile walls activated by a stepping motor allowed us to conduct the deflation experiments during continuous extension. Combination of deflation and extension leads to the creation of two lateral depressions and one axial trough. The lateral depressions are controlled by normal faults, while the central trough is delineated by reverse faults. During balloon deflation, tectonic extension is accommodated away from the axis due to the presence of the ductile silicone layer, whereas the deflation of the axial reservoir is accommodated by the collapse of the overlaying brittle crust. This accounts (1) for the development of axial troughs as collapse calderas, and (2) for the ubiquitous formation of lateral grabens of the flanking tectonic province in response to the deformation of the crystal mush.

Faulting pattern along slow-spreading ridge segments: a consequence of along-axis variation in lithospheric rheology

Abstract We present here results from an analogue model designed to test the relative influence of along-axis variations of the crustal thickness and of the thermal structure of the lithosphere on the geometry and on the faulting pattern of the axial rift of a slow-spreading ridge. Rheologically calculated layered models are employed, using quartz sand and silicone putty as analogues of the brittle and ductile components of the lithosphere, respectively. Two parameters have been analysed in detail: (1) the thickness of the brittle layer, and (2) the viscosity of the ductile layer. This study shows that the thickening of the brittle layer and the increase of the viscosity of the ductile layer bring about a widening of the axial valley associated with an increase of the number of faults. A decrease of the depth of the axial valley and of the vertical throw of the faults is observed with an increase of the viscosity of the ductile layer and a decrease of the thickness of the brittle layer. These results are consistent with observations along Mid-Atlantic Ridge segments. For segment ends, the fault pattern obtained in the models is similar to that described on both sides of the axis for segments bordered by a zero-offset discontinuity, and on outside corners for segments bordered by a lateral offset discontinuity. Our results suggest that the viscosity of the ductile layer plays a more important role in the fault pattern than the thickness of the brittle layer. The influence of segment length, offset length and temporal variation in thermal input could explain the more or less important along-axis variation in the deformation pattern observed along segments of slow-spreading ridges.

Analogue modeling of faulting pattern, ductile deformation, and vertical motion in strike-slip fault zones

The relationship between faulting, ductile deformation, and vertical displacement in provinces of strike-slip faults were analyzed using analogue modeling. The described experiment considered a two-layer, small-scale model for the lithosphere, set on a low-viscosity fluid that allowed free isostatic compensation. The development of the faults and their orientation follow generally Riedels model, but fault spacing depends on the thickness of the brittle layer, and lateral displacement of the brittle layer was associated with series of deep, elongated depressions that developed along the trace of the principal strike-slip fault. The depressions correspond to local thinning of the brittle layer, and the amount of thinning is more than 60% in places. The underlying ductile layer displayed two types of superimposed deformation, namely a series of tight, parallel folds, occurring on top of elongated domes. The folds are attributed to the compressive component of the strike-slip displacement, and the updoming to the extensional component. The elongated domes are located beneath the superficial depressions, and the deeper troughs underlie shallow uplifted structures in the deformed band. Significant vertical motion was observed along faults that are considered as perfectly strike-slip faults, according to the classical Riedel model. The thinning of the brittle layer, and the deep deformation of the ductile layer, are in good agreement with actual examples of pull-apart basins and elongated swells in large strike-slip zones. A new structural pattern developed during strike-slip shear is proposed, relating the geometrical relationships between faults, folds, and elongated domes to the stress axes.

Deformation processes at fast to ultra-fast oceanic spreading axes: mechanical approach

Abstract The morphology of fast to ultra-fast oceanic spreading ridges such as the East Pacific Rise (EPR) is characterized by an axial dome, 5–10 km wide, culminating at 300–500 m above the surrounding seafloor. This dome is bounded by lateral grabens that develop systematically 2 to 6 km apart from the spreading axis. A large summit trough, 200 m to 2 km wide, locally notches the axial high, only where the dome is inflated, indicative of a time-average robust magma supply. This summit trough is thought to represent an elongated axial summit caldera (ASC) created as a result of the subsidence of the top of the axial magma chamber (AMC). Such subsidence is likely caused by a temporary decrease in melt supply into the shallow magma reservoir suffering continuous regional extension. Analog experiments using small-scale modeling have been performed in order to better constrain the tectonic evolution of the axial region. The experimental apparatus includes an elongated balloon filled with water as an analog of the magma reservoir set in a central groove in a table. It is capped with a silicone layer representing hot rocks below the brittle–ductile transition and is covered by a sand layer representing the brittle crust. The experiments integrate withdrawal of the balloon and extension at the boundary of the model by the mean of two mobile walls. Three experimental setups allowed us to study independently the mechanical parameters controlling the axial tectonic evolution: extension without withdrawal, withdrawal without extension, withdrawal and synchronous extension. We show that the morphology of the EPR axis can be considered as the result of both horizontal and vertical movements. Two symmetrical lateral grabens develop on both sides of a non-deformed axial dome when single extension is applied to a model with a thin silicone layer. Normal faults of the lateral grabens are rooted on two divergent velocity zones (DVZs) located on the edges of the groove. This situation is regarded as an analog of the natural case where the top of the AMC acts as a stress-free boundary that fails to transmit the extensional stresses to the upper brittle layer. An important deflation of the balloon without extension results in the creation of a central collapse trough limited by reverse faults. During synchronous extension and withdrawal, the initiation of the lateral grabens is favored by a balloon deflation, even if such deflation is unable to generate a superficial collapse. This last case is considered as representative of the evolution of EPR segments showing little variations in melt supply into the AMC. Higher deflation rates under continuous extension correspond to EPR segments undergoing strong variations in melt supply. In such experiments, the lateral grabens are created together with a central collapse trough developing in a way similar to that of experiments involving only balloon deflation. Finally, we show that DVZs located at the brittle–ductile boundary are the key mechanical elements which may explain the structural evolution of the axial region of fast to ultra-fast spreading ridges. The distance from axis and the width of the DVZs directly control the location and the distribution of the lateral grabens.

Geoelectrical Tomography Investigating and Modeling of Fractures Network around Bittit Spring (Middle Atlas, Morocco)

Direct current Resistivity (DCR) method was carried out to characterize the hydrogeological connection between the Tabular Middle Atlas (TMA) and the Sais Basin. The TMA is one of the most important aquifers in northern Morocco that supplies the deep aquifer of the Sais Basin. Electrical resistivity tomography (ERT) survey was focused on the Bittit area that is one of the most important outlet discharges, and it is representative of the relations between the TMA and the Sais Basin. The high resolution capabilities of the electrical tomography were used to define the geological draining features in the framework of water paths from the TMA to the karstic springs. The synthetic data were calculated for the similar model expected in field data inversion and inversion result of these synthetic data used as a guide for the interpretation of the inverse data resistivity sections. Joint interpretation of geophysical, geological, structural, and synthetic simulation data allowed identifying a conductive horizontal shallow layer overlying two subvertical families of fractures of NE-SW and NW-SE directions. This result leads to propose hydrological behaviour of water from the Tabular Middle Atlas and the Sais Basin at the Bittit Spring, which takes into account for both horizontal flows through stratification joints or karst and through subvertical fractures.

Deglaciation and volcano-seismic activity in Northern Iceland: Holocene and early Eemian

The North Volcanic Zone of Iceland was unglaciated during most interglacials. Subsequently, the region was covered by the Weichselian ice cap. A widespread interglacial complex, the Syđra Formation, has been mapped in this zone. It covers probably O.I.S.5e, 5d and 5c. Its formation and preservation are discussed in terms of rift and volcanism activity, in interrelations with the former deglaciation. A topographic bulge, presumed of glacio-isostatic origin, limited the downstream drainage of the Jökulsa a Fjolum river enabling the interglacial sedimentation and the excavation of one of the canyons of Dettifoss. Effusive volcanic activity in the rift is important prior to the Syđra 4 unit in association with an early abrupt event (SY2: Syđra ash), related to a phreato-magmatic eruption at the eastern hyaloclastite ridge or from the Askja volcano and to jökulhlaup events. It corresponds probably to ash Zone B as defined by Sejrup et al., (1989) on the Northern Iceland shelf. The previous activity of hyaloclastite ridge is recorded during the Marine Isotope Stage 6 (MIS 6 = Saalian) and its deglaciation, a younger effusive event is dated at 80 ka. The Interglacial paleo-seismic region is similar to the present one; during deglaciation, the seismic zone is widened, up to 60 km to the East. Continuous micro-seismicity related to dyke intrusion and effusive or phreato-magmatic eruptions develop at the onset of deglaciation. It is discrete during the full interglacials, and most intense during pyroclastic eruptions. A comparison with the Late Glacial/Holocene deglaciation is provided in the same region.