Henri Got

From subduction to transform motion: a seabeam survey of the Hellenic trench system

Abstract Preliminary results of a multi-narrow beam survey of the Hellenic trench system, in the Eastern Mediterranean, are presented. The southwestern Ionian branch is divided in small basins, partly filled with Pleistocene sediments. The morphology suggests that the basins are deformed by a compressional stress acting roughly perpendicularly to the trench along N50°E. This direction is the direction of the regional slip vector of the shallow thrust-type earthquakes. The structure of the southeastern Pliny-Strabo branch is quite different. Narrow en-e´chelon slots, oriented N40°E, have been mapped within the main troughs oriented N60°E. The regional earthquake slip vector is also oriented along N40°E. We conclude that the Hellenic trench system is an active subduction system, dominated by thrust along the Ionian branch and by transform motion along the Pliny-Strabo branch.

Valencia Fan (northwestern Mediterranean): Distal deposition fan variant

Abstract The Valencia Fan, one of the larger deep-sea depositional systems in the western Mediterranean Sea, developed in the large depression between the Valencia Trough and the Balearic Basin Plain. Six main lithoseismic units are identified and sedimentary processes inferred from 6000 km of sparker profiles. This fan is largely formed by channelized and irregularly stratified units. The sedimentary processes controlling the development of these units include channelized sediment flows that evolve downfan into sheet flows. Three fan depositional provinces are differentiated on the basis of the relative proportions of lithoseismic units and the inferred sedimentary processes. Regularly stratified seismic units predominate in the non-fan environments. These units are dominated by fine-grained deposits resulting from hemipelagic settling and overbank flows from turbid currents. Distal flows from the continental slopes of the Iberian Peninsula and Balearic Islands also contribute materials for the development of these environments. The wavy units flanking the upper fan may have resulted from migrating sediment waves or limited mass-displacement of the stratified units, while the transparent units are attributed to extensive mass-flow. The Valencia Valley is largely an erosional feature across which sediments from several source areas bypassed to the distal, deep-sea depositional system of the Valencia Fan. Deposition begins at the mouth of the valley beyond which there is a break in slope. The fan migrated upslope and filled the Valencia Valley from the distal sectors. This evolution has been controlled by the interplay between sedimentary processes active in the fan growth and large-scale geological factors related to the structural framework of the study area.

The Ebro and the Rhone deep-sea fans: First comparative study☆

Abstract Bathymetric and continuous-seismic profiles enable us to define the main morphological and structural outlines of the Ebro and Rhone deep-sea fans. Despite great differences of morphology and volume, these two fans are characterized by a similar structure with a 500 milliseconds thick series of superposed lenticular acoustic units lying above more homogeneous sediments. These units are tentatively attributed to channel-infilling, associated with overbank deposits shifting through time. Nevertheless, some of them may represent large sedimentary slides, especially off the Ebro river.

Emplacement of the recent sedimentary cover and processes of deposition on the Matapan Trench margin (Hellenic Arc)

Abstract High-resolution seismic surveys across the southern Peloponnesus margin help to define the configuration and geometry of the unconsolidated sedimentary cover. On the basis of acoustic character, structural framework and results of DSDP cores, two sedimentary units are distinguished: a well-stratified Quaternary unit and an acoustically transparent Pliocene series overlying a deep acoustic unit of different nature and age. The Plio-Quaternary series appears highly discontinuous along most of the margin. The slope displays a series of basins where, locally, sediment thicknesses exceed 1000 m. These basins, separated by structural ridges generally devoid of sediment, are often connected to the trenches by canyons. The mechanism of sedimentation termed cascade feeding consists in a succession of depositions and of removals of sediment from basin to basin until the trench basins are reached. The major physiographic features involved in this mechanism (slope basins, canyons, ridges, depressions, trenches) are directly controlled by structural trends oriented N140 and N70 related with the neotectonic evolution of the Hellenic Arc.

Sedimentary Processes in Mediterranean Deltas and Shelves

Recent studies enable us to compare the geographic and physiographic settings, general structural configuration, emerged delta evolution, hydrodynamic regime, suspended transport processes, and geometry of recent shelf deposits seaward of the four major Mediterranean deltas: the Rhone, Ebro, Po, and Nile. This synthesis indicates that some features of the deltas and adjacent continental shelves appear to have remained constant during the last high sea-level stand (Holocene) around the Mediterranean. These include prismatic geometry of Holocene deposits; nearshore smectite-rich prodelta muds; multilayer system of suspended sediments on the continental shelf, with a bottom nepheloid layer; and longitudinal bottom and surface currents. Sedimentation depends on the balance between climatic-eustatic changes, river discharge, and current dispersal.

Ebro Fan, Mediterranean

The Ebro Fan System consists of en echelon channel-levee complexes, 50 x 20 km in area and 200-m thick. A few strong reflectors in a generally transparent seismic facies identify the sand-rich channel floors and levee crests. Numerous continuous acoustic reflectors characterize overbank turbidites and hemipelagites that blanket abandoned channel-levee complexes. The interlobe areas between channel complexes fill with homogeneous mud and sand from mass flow and overbank deposition; these exhibit a transparent seismic character. The steep continental rise and sediment “drainage” of Valencia Trough at the end of the channel-levee complexes prevent the development of distributary channels and midfan lobe deposits.

Detailed morphology, structure and main growth pattern of the Rhône deep-sea fan

Abstract A sea-beam survey of the Rhone deep-sea fan and its vector canyon shows unexpected morphological features such as striking similarities with continental landforms encountered especially in alluvial plain areas or desertic countries. The structure of this fan, as shown by continuous seismic profiles, results from the stacking up of numerous lenticular acoustic units displaying specific seismic characters. It is suggested that the building of the fan began as early as the lower Pliocene with a growth pattern tightly associated with climatic fluctuations.

Subduction in the Hellenic Trench: probable role of a thick evaporitic layer based on Seabeam and submersible studies

Summary A Seabeam survey of the Hellenic Trench in 1978 was followed by a submersible survey in 1980. Fifteen dives were completed between 1500 and 3000 m covering 58 km on the bottom and obtaining 48 samples. Thus, the Hellenic Trench is probably now the most intensively studied trench as previous work included seismic reflection, dredging, coring, bottom photography, side-scan sonar and drilling. Because of the high sedimentation rate and the relatively shallow depth, we did not expect outcrops except along faults and in canyons. Active tectonics with numerous faults and folds have been observed. But we have also discovered vertical cliffs with hard rock exposure over a depth range of 1000–1500 m. In this paper, we discuss the nature of the hard rock outcrops observed both on the inner wall and on two hills that we interpret as diapirs. We conclude that at least part of these hard rocks are probably Messinian evaporites and concentrate on the tectonic role of this evaporitic layer in the Hellenic subduction. It is likely that the evaporites do not subduct but tend to pile up in the trench, to form a floating evaporitic basin.

Seismic Structure of Southwestern Peloponnesus Continental Margin

About 3,000 km of geophysical profiles (sparker and air gun) were shot off southeastern Peloponnesus during several cruises in 1976-77 as part of a general study of the active Hellenic margin. These data, supplemented by the results of previous surveys, provide a basis for mapping the overall structure of the area. The main margin depocenters lie on the upper slope where they are represented by tectonically bounded plateaus and basins. The lower to middle slope is an area of little sedimentation but intensive tectonization, thus an area of numerous basement ridges and zones of deformed sediments. In contrast, the base of the margin is characterized by discontinuous, deep, and sediment-filled depressions--the North Matapan Trough and the South Matapan Trough, separated by region of prominent topography, the Matapan Seamounts. Two main trends (N 140° and N 70 to 80°) control the structure of the area; both can tentatively be interpreted in the light of compression originated by subduction acting along the Ionian margin.

Hydrodynamic study of the furrow of the Petit Rhône canyon

Abstract A Seabeam survey of the Petit Rhone canyon and deep-sea fan, in the northwestern Meditteranean Sea, shows the existence of a furrow, which resembles those of some terrestrial rivers. The hydrodynamic regime of this furrow is discussed, taking into account some morphological features, structural characteristics and calculated volume of sediments expected to have been transported down-slope.