Maria Sachpazi

Perturbation to the lithosphere along the hotspot track of La Réunion from an offshore-onshore seismic transect

A 250 km long NE-SW lithospheric transect spanning the 40 km wide island of La Reunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high- velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island-crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material.

Rift structure, evolution, and earthquakes in the Gulf of Corinth, from reflection seismic images

Abstract Continental extension is forming the Gulf of Corinth across the strike of earlier Alpine evolution. Here, we present the first deep reflection sections with pre-stack depth-migration processing across the deep basin of the Corinth active rift, which image structures unpredicted by current models. Resolving the infill as a pile of layers that are broken by faults allows one to follow their subsidence and deformation history. Variation among the profiles suggests that southern normal faults control the rift in a time progression from the east towards its western tip. On the central, Derveni–Itea transect, a 3-km widening of the basin accrued since the initiation of this control that is marked by an unconformity between the two main sedimentary units. This is estimated to have occurred 0.5–0.6 Myr ago, assuming the glacio-eustatic sea-level changes have controlled the stratigraphy of sediments deposited as a succession of layers on the subsiding hangingwall, as they did on the southern footwall in forming the famous flight of marine terraces of Corinth. A roll-over anticline and crestal collapse graben are diagnostic of the control by a normal fault of dip varying with depth. The deeper low-angle part of this bi-planar fault is indeed imaged as a reflector in the basement. The occurrence of the collapse with a breakaway at the steep southern basin-bounding fault of the hangingwall slab can be estimated 0.12–0.2 Myr ago, with a marked increase in extension rate that brought it to its present fastest value over 10 mm/yr. The low-angle part of the active fault might also have controlled earlier evolution upslope and in the basin. When compared with inferences from earthquake studies, this low-angle active fault may not appear to be seismogenic but may participate to the seismic cycle, possibly in a conditionally stable regime. Active faults seen as sea-bottom scarps merely accommodate deformation of its subsiding hangingwall. The footwall of the low-angle faults, which current seismicity shows to be in extension, appears then as being pulled out from beneath the rift, in a motion towards the rolling-back slab that causes the Hellenic subduction retreat.

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.

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.

Seismological and SAR signature of unrest at Nisyros caldera, Greece

Abstract Nisyros island, a Quaternary volcanic center located at the SE of the Aegean Volcanic Arc, has been in the past characterized by periods of intense seismic activity accompanied sometimes by hydrothermal explosions, the last one being in 1887. The recent long lasting episode of unrest (1995–1998) in the area is the first instrumentally documented providing information on the behavior of the volcano. Evidence from seismicity and SAR interferometry suggests that the presently active part of the Kos–Nisyros volcano-tectonic complex is located at the NW coast of Nisyros island defining an area much smaller than the whole volcano-tectonic area. Seismicity patterns vary both temporally and spatially consistently with different rates of vertical ground deformation inferred from SAR interferometry. These observations help us to discuss the different elements controlling the behavior of the volcanic system such as: the existence, location and timing of magma chamber inflation, the occurrence of tensile failure at the boundaries of the chamber and the possibility of magmatic fluids being expelled to form a shallow magmatic intrusion, the seismic failure and migration of hypocenters indicating shallow magma transport.

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.

Seismic coupling and structure of the Hellenic subduction zone in the Ionian Islands region

Abstract The western Hellenic arc has been commonly considered as a largely aseismic subduction zone, from the comparison of a small rate of shortening derived from the seismic moment release, with a large rate of convergence inferred from geology. Complete seismic coupling would instead be expected from models that consider a control by plate tectonic forces, because of the trenchward velocity of the Hellenic–Aegean upper plate now confirmed with GPS measurements. In the region of the Ionian Islands, a subduction interplate boundary has been recently imaged and its seismogenic downdip width suggested to be moderate, from reflection seismic profiling and local earthquake tomography. In the appropriate model for such an earthquake source region, which considers a single interplate fault and takes into account these features, the moderate seismic moment release is found consistent with complete seismic coupling of this subduction. The shallow downdip limit of the seismogenic zone can be interpreted as due to the interplate boundary being overlain there by the ductile deeper crust of the orogenically thickened Hellenides.

Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

The variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation.

Segmented Hellenic slab rollback driving Aegean deformation and seismicity

The NE dipping slab of the Hellenic subduction is imaged in unprecedented detail using teleseismic receiver function analysis on a dense 2-D seismic array. Mapping of slab geometry for over 300 km along strike and down to 100 km depth reveals a segmentation into dipping panels by along-dip faults. Resolved intermediate-depth seismicity commonly attributed to dehydration embrittlement is shown to be clustered along these faults. Large earthquakes occurrence within the upper and lower plate and at the interplate megathrust boundary show a striking correlation with the slab faults suggesting high mechanical coupling between the two plates. Our results imply that the general slab rollback occurs here in a differential piecewise manner imposing its specific stress and deformation pattern onto the overriding Aegean plate.