Alfred Hirn

Roots of Etna volcano in faults of great earthquakes

Abstract Results from several seismic methods allow us to sketch the deep structure of Etna and its Ionian margin. Under Etna a volume of high velocity material is found in a structurally high position; the emplacement of this suggests spreading of the surrounding medium. Just offshore, down-to-the-east normal faults penetrate through the upper crust. The deeper crustal structure beneath appears upwarped from the basin towards Etna. Juxtaposed with the crust of Sicily, a thinner crust reaches from the Ionian Basin under Etna, and the mantle is upwarped. In such a structure, magma can then be viewed as a melted lens capping a mantle upwarp at shallow depth, rather than in an intracrustal chamber. This reduces the conflict between estimates of its volume from excess output of volatiles and short residence times. A link in time is indicated between volcanic and seismic activity at a large scale: over the millennium the reported ends of episodes of high output rates of magma are followed by the reported occurrences of magnitude 7 + earthquakes which caused destruction in southeastern Sicily. Several steep active normal fault have been imaged to a depth of 10 km the crust up to 30 km offshore of the cities of Catania and Augusta, which may be fault planes for such large earthquakes. They expand and prolongate the system of the Timpe faults on the eastern flank of Etna, thus linking large-scale tectonics offshore with the volcano. Etna developed together with normal faulting, upwarp, and spreading during the recent evolution of the former Ionian subduction. Activation of the material at depth at the lateral edge of the slab, by vertical motion with extension above, could produce the peculiar type of Etna magmatism.

Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution

Abstract Crustal imaging could be achieved on normal-incidence reflection profiles offshore eastern Sicily by using industrial-grade reflection seismic with improved marine sources. Thick recent sediments, a reflective pile including the Mesozoic deposits, a transparent upper crust, and a band of low frequency reflections attributed to the lower crust, are imaged in the seismic sections. The structure of the crust and its thickness show features inherited from the Mesozoic evolution as a passive margin, by which Ionian basin crust was formed around the Hyblean continental promontory of Africa to constitute the southern plate in the later convergence with Europe. The seismic images are also marked by the lithospheric deformation due to the Neogene overriding of the northern part of this paleomargin by the Calabro-Peloritan block of European continental crust. This transpressive motion may have been guided along the northern part of this paleomargin where the seismic profiles evidence a hinge line between the northward upslope of the Moho of that old passive margin and its downslope to the present slab under the Tyrrhenian Sea. Etna volcano is located at the intersection of this mantle upwarp by a zone of active sea-bottom normal-faults, which cut across the formerly constructed compressional belt. The onset of its volcanic activity is roughly coeval with that of the cessation of interplate thrusting and could hence be related to a change of the coupling of the Ionian slab. This slab is now probably disconnected from the overriding plate and rolled back in front of the expanding hot Tyrrhenian asthenospheric dome with the mobilisation of a viscous mantle material at depth. An active lithospheric fault is here imaged which cuts over more than 100xa0km into the Ionian basin. The fault runs from the Tyrrhenian margin in the SSE direction of Etna updip of the southwestern lateral edge of this slab, leaving north-eastward the extruded Calabrian block and on its south-westward edge the uplifted crust and mantle structures of Etna. Along it, the crust, including the Mesozoic and deeper layers, has sagged vertically in the segment in front of the slab, with a finite throw across the fault increasing from the basin towards Etna.

Implications of the seismic structure for the orogenic evolution of the Pyrenean Range

Abstract The dynamic evolution of the Pyrenees is discussed in the light of geophysical data. Recent deep seismic sounding have revealed the crustal structure of the Pyrenees which is used to test the different evolutionary models proposed until now. The crustal thickness of the Paleozoic Axial Zone (PAZ) and the North Pyrenean Zone (NPZ) differ by more than 10 km, ranging from about 30 km in the NPZ to 40–50 km in the PAZ. The transition from PAZ to the NPZ, identified at the surface as the North Pyrenean Fault (NPF), is sharp at depth and marked by a vertical step, at least in the eastern half of the range. The NPZ is characterized by additional throws and dips of the Moho in the east whereas in the west a heterogeneous middle to lower crust is encountered, with high velocity anomalies. The seismic results suggest that the PAZ and the NPZ belong to different plates, the NPF being the plate boundary. These results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following NNE-SSW directions. They rather suggest that after a period of extension, two main orogenic events took place: a phase involving shearing and thinning which affected mainly the present-day NPZ and a later compressive phase which explains the building up of the chain, the thickening of the crust and the enhancement of a pre-existing difference in crustal thickness between the European and Iberian plates.

Internal structure of Piton de la Fournaise volcano from seismic wave propagation and earthquake distribution

Abstract Arrival times of seismic waves from local earthquakes are inverted for both locating the source and defining the 3-D velocity heterogeneity of Piton de la Fournaise. The lateral heterogeneity of the 2632 m high edifice is resolved as a high-velocity plug, 1.5 km in diameter, surrounded by a low-velocity ring, which may be interpreted as due to the construction of Fournaise on the flank of the older volcano Piton des Neiges. Wave mode conversion detected on three-component seismograms provides evidence for boundaries of contrasted velocities. Pre-eruptive swarm earthquakes cluster in the high-velocity zone, under the Dolomieu summit crater. Low strength and cohesion of the surrounding material account for the lack of seismicity for the final 1–3 km radial flow of magma to the vents in Enclos Fouque. Beneath the high-velocity plug the existence of a body with low velocity for P, and even for S, waves is well constrained. However, the walls and base are poorly defined because of the lack of deep earthquakes for sampling. The few earthquakes that are located in this depth region usually occur at a depth of around 1.5 km below sea level in the region of the cone. This can be considered providing the upper constraint on the lower limit of the aseismic part of the low-velocity body. The coincidence in time of their occurrence with the swarms above sea level and the eruptions suggests magmatic activation of the low-velocity aseismic volume 1.5 km below sea level under the high-velocity plug of the cone. Further down, the concentration of seismicity in two swarms, between 2 and 4 km, under the eastern flank does not allow the structure to be sampled effectively.

Deep structure of the Armorican Basin (Bay of Biscay): a review of Norgasis seismic reflection and refraction data

The Bay of Biscay is bounded to the north by the North Biscay margin, which comprises the Western Approaches and Armorican segments. In the 1970s and 1980s, most researchers considered this margin typical of a non-volcanic passive margin: it is characterized by a striking succession of tilted blocks beneath which occurs the S reflector and the continent–ocean boundary is abrupt. This paper examines the Armorican segment and is based on a study of all early seismic profiles together with new multichannel reflection and refraction seismic data (Norgasis cruise). An important result is the discovery of a 80 km wide ocean–continent transition zone that coincides with the Armorican Basin (a deep sedimentary basin). It is characterized by a high-velocity lower-crustal layer (7.4–7.5 km s−1) overlain by sediments. The other results are: (1) the main crustal thinning occurs exclusively under the narrow continental slope; (2) the tilted blocks and the S reflector are observed only at the base of the continental slope in the narrow domain called the ‘neck area’; (3) the North Biscay Ridge is a large oceanic plateau present only off the NW Armorican margin rather than a long ridge elongated off the whole North Biscay margin.

Seismic structure and the active Hellenic subduction in the Ionian islands

In the region of the Ionian Islands of western Greece, the active margin of the Hellenic domain passes from oceanic subduction in the south to continental collision in the north, linked by the right-lateral Cephalonia transform fault. A slightly landward dipping interface revealed at 13 km depth by a single previous line in the channel between Cephalonia and Zante has been suggested as the interplate subduction boundary. New marine multichannel reflection profiles and OBS refraction and wide-angle reflection data confirm the reflector as a regional feature. These data evidence its extension to the south, where large, low-angle thrust earthquakes occur offshore to Zante. The new profiles establish a coincidence between the focal depths of these large subduction events and the imaged bright reflective level, confirming its tentative interpretation as the interplate boundary, which generally appears with a positive reflection polarity. In this context, the Ionian Islands outcrop corresponds to a shallowing of the interplate boudary from south to north. In the south, offshore Zante, the interplate boundary comprises a stratified zone that may be considered as the sedimentary cover of the Ionian Basin oceanic-like crust, which forms the lower plate here. The shallower position and single-cycle reflection character of the interplate further north suggest that the lower plate could there be the Apulian paleomargin to that basin.

Stress, failure and fluid flow deduced from earthquakes accompanying eruptions at Piton de la Fournaise volcano

Abstract In the present episode of eruptive activity, evidence from seismicity for sustained magma inflow from depth into the edifice of Piton de la Fournaise is lacking. Pre-eruptive main deformation and shallow seismicity help to identify very small volumes of magma that are in motion beneath the rim of the Dolomieu summit crater, and oriented along the azimuth of the future vents. Small magma pockets may reside in the cone above sea level, or may be expelled repeatedly, due to crystallisation in a small, low-velocity, aseismic region below sea level under the high-velocity central plug of the cone in which pre-eruptive earthquake swarms are located. In cross-section the hypocentres define two steep sheets diverging from the aseismic zone at sea level towards 1.5 km above sea level (or 1 km beneath the 2632 m high cone). However, failure induced by increased pressure in the suggested chamber does not account for the observed focal mechanisms. The occurrence and timing of magma transport are attested by eruption, and seismic activity may be related to magma transport. Focal mechanisms document strike-slip, not normal faulting or tensile failure. Vertical propagation of the edge of a feeder dike may enhance strike-slip motion above the edge, in a region where effective normal stress is decreased by thermally induced groundwater flow. The strike-slip mechanisms could also be caused by a tensile-shear widening of the horizontal section of vertical conduits. Fournaise strike-slip earthquakes occur in two orientations, with P axes orthogonal between them, within a single pre-eruptive event. Earthquakes are distributed in the same volume but mechanisms switch from one to another type systematically with time, indicating a reversal of stress conditions. The orientations of P axes with respect to the epicentral trend suggest that in the later parts of events leading to eruptions, a compression of the medium occurs after a dilation in the first part. The activated zone might respond successively to the arrival and the departure of the magma on its way from the reservoir at depth to the vent, radial to the cone.

SINGLE-BUBBLE MARINE SOURCE OFFERS NEW PERSPECTIVES FOR LITHOSPHERIC EXPLORATION

Abstract We present deep penetration seismic time sections obtained in the last two years with the “single-bubble” pulse generating method using marine seismic airguns. Whole crustal penetration and resolution have been obtained on the Ionian quasi oceanic basin and its margin, on the continental crust of the Aegean Sea, on the continental margins, and oceanic basin of the Gulf of Biscay. We show that this method is more efficient than others in radiating the lower frequencies returned by deep structure, without loosing resolution. Hence it allows deep crustal imaging from relatively low-power academic ships, and if the method is applied to powerful industrial vessels, it may open the subcrustal lithosphere to imaging.

North Aegean crustal deformation: An active fault imaged to 10 km depth by reflection seismic data

Three multichannel seismic profiles imaged a normal fault to at least 10 km depth in the North Aegean Trough and Thermaikos basin. The fault is active and recent, forming a scarp at sea bottom and crossing the Quaternary deltaic front on the northern slope of the trough controlled to the south by the North Anatolian fault. Prestack depth migration imaged the fault as a seismic reflector cutting steeply across the sedimentary rocks and flattening in the basement. From the seismic image, the N100°E strike of the fault scarp, and the orientations of the three profiles, the true fault dip is constrained to an average 20° in the basement, a low-angle dip. The throw and age estimated from the geometry in the sedimentary rocks document recent onset of the motion that must have occurred at a high rate. Both the direction and the rate of slip are consistent with the instantaneous motion as measured by space-based geodesy, which shows the fault to be forming by pure normal slip. Large earthquakes that have occurred in the basin may relate to such normal faults, whereas the North Anatolian fault with its current strike-slip earthquakes appears to slip here under low resolved shear stress.

A TRAVERSE OF THE IONIAN ISLANDS FRONT WITH COINCIDENT NORMAL INCIDENCE AND WIDE-ANGLE SEISMICS

Abstract Marine vertical reflection profiling with a powerful airgun source, augmented with a few wide-angle seismometer stations on land, has been applied along a 180-km line across the presently active deformation front of the subduction of the Ionian Sea plate beneath the western Hellenides. East of the Ionian islands, there is limited evidence for reflectivity down to 23 km interpreted as the base of a rather thin continental-type crust. Under the western slope of the islands, a major normal-incidence reflector dips eastward, first gently to the west of the islands, then more steeply under them. This reflector may be extrapolated southwestwards beneath a bulge that is thought to represent the modern pre-Apulian front located over the subduction zone. The continuity, signature, and geometry of this reflector suggest that it may act as the lower limit of the western Hellenides where they override the Ionian Sea quasi-oceanic crust, rather than just an intracrustal interface of a pre-Apulian crust. The location of the previous deformation front, the Ionian thrust proposed in previous models, can be constrained by the new seismic data. The new data raise the possibility of a larger area of evaporite mobility than previously considered, insofar as active block motion apparently related to halokinesis is recognized west of the Zakynthos anticline. Diapirism, decollement, and westward-directed over-thrusting in the pre-Apulian crust may have been brought on by Late Pliocene-Quaternary reactivation of regional extension associated with the separation of the Peloponnesus from northern Greece as it was captured by the Anatolian-Aegean rotation and associated also with the fast clockwise rotation of the Ionian islands as they were sheared off the Apulian domain to the southwest by the initiation of the Kefallinia transform.