Eric Chaumillon

From Oblique Subduction to Intra-Continental Transpression: Structures of the Southern Kermadec-Hikurangi Margin from Multibeam Bathymetry, Side-Scan Sonar and Seismic Reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.

FROM FORELAND TO FOREARC DOMAINS : NEW MULTICHANNEL SEISMIC REFLECTION SURVEY OF THE MEDITERRANEAN RIDGE ACCRETIONARY COMPLEX (EASTERN MEDITERRANEAN)

Abstract The Mediterranean Ridge of the Eastern Mediterranean Sea is a huge accretionary wedge emplaced as a consequence of the Africa-Aegean plate convergence, that has a rate estimated at ~40 mm/yr. Processing of recent multichannel seismic data and integration with previous results (seismic profiling, swath bathymetry, sonar) have facilitated the study of the overall deformation pattern of the area which includes three distinct major structural domains facing various forelands: 1. (1) The outer domain is bounded southward by the present-day wedge toe and towards the north by topographic slope breaks and clear changes in deformational style. Beneath the Ionian and Levantine Mediterranean Ridge branches the base of Messinian evaporites seems to act as a major decollement level. 2. (2) The central, or crestal, domain, the shallowest one, shows evidence of mud diapiric and mud volcano activity and is overthrusting northward over the inner domain. Facing the Libyan margin, this area, which displays spectacular mud volcano fields, appears much wider and shallower. 3. (3) The less-deformed inner domain is bounded northward by the Hellenic trench system. Beneath its Ionian inner area, seismic velocities characteristic of continental crust have been recorded, while within its Levantine region, it is made of a series of disconnected and uplifted basement blocks (Strabo seamounts). Along the Matapan and Pliny trenches structures which could be caused by transpressive deformation are locally observed. We believe that both the north-south structural arrangement and the clear west—east lateral variations of the Mediterranean Ridge relate to its progressive collision against the Libyan margin promontory. This event, which may have initiated ~5–6 Myr ago, may have induced: (a) a differentiated paleogeography of Messinian evaporites (later available for tectonic accretion) leading to the further lateral structural variations of the outer domain; and (b) strain partitioning between the central Mediterranean Ridge and the Hellenic trench system.

Deformational styles of the eastern Mediterranean Ridge and surroundings from combined swath mapping and seiemic reflection profiling

Abstract Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north–south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en echelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean–Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area.

Deformation of the western Mediterranean Ridge: Importance of Messinian evaporitic formations

Abstract Multichannel seismic data were acquired across several areas of the Mediterranean Ridge (M.R.) in the eastern Mediterranean Sea during the MCS Prismed survey (March 1993). The M.R. is a wide, high-standing and elongated swell, emplaced between the southern Aegean and Africa as a consequence of the long-term convergence and the ongoing collision between Europe and Africa. Sedimentary piling and associated deformation have built a thick deformed sediment wedge, the internal geological structures of which remain poorly understood. Processing of two MCS lines (PM 02 and PM 03) across the western branch of the Mediterranean Ridge reveals that Messinian evaporite-bearing formations, which were deposited within the deep Ionian oceanic basin 5 my ago, play an important part in the present structural deformation of the M.R. accretionary wedge. Within the outer M.R., the Messinian “lower evaporites” are believed to act as one of the main decollement levels, while the top of these formations may operate as a disharmonic layer below thin Plio-Quaternary sediments. The present-day M.R. toe appears to be affected by ductile-flow deformation. However, most of the accreted outer wedge is believed to have resulted from the progressive piling up and associated thrusting of Messinian and overlying Plio-Quaternary sediments. Toward the northeast (i.e., toward the central M.R. domain), a major backthrust zone indicates a structural boundary between the outer, wide and convex M.R. and its inner, rather flat, domain, which is inferred to act as a backstop.

Incised-valley morphologies and sedimentary-fills within the inner shelf of the Bay of Biscay (France): A synthesis

This study is a first synthesis focused on incised-valleys located within the inner shelf of the Bay of Biscay. It is based on previously published results obtained during recent seismic surveys and coring campaigns. The morphology of the valleys appears to be strongly controlled by tectonics and lithology. The Pleistocene sedimentary cover of the shelf is very thin and discontinuous with a maximum thickness ranging between 30 and 40 m in incised-valley fills. Thus the incised bedrock morphology plays a key-role by controlling hydrodynamics and related sediment transport and deposition that explains some variations of those incised-valley fills with respect to the previously published general models.

Morphological evolution and time-varying bedrock control of main channel at a mixed energy tidal inlet: Maumusson Inlet, France

Abstract Maumusson Inlet, located on the French Atlantic coast, connects the Atlantic Ocean with the Marennes–Oleron tidal bay. The tidal range (2–6 m) and wave climate (mean height 1.5 m) place this tidal inlet in the mixed energy, tide dominant, category of Hayes [(1979) Barrier island morphology. In: Leatherman, S.P. (Ed.), Barrier Island, Academic Press, New York, pp. 1–28]. An innovative method, combining high quality bathymetric data (nine accurate Digital Elevation Models since 1824) with a very high seismic resolution, demonstrates a major tidal inlet evolution from 1824 onwards and its dramatic acceleration since 1970. The chronology of those morphological changes suggests strong coupling between the location of the tidal channel and the behaviour of the adjacent shorelines. The recent shoaling and migration of the inlet channel can be attributed to a decrease in tidal prism due to the filling in sediment of Marennes–Oleron Bay. Seismic data give evidence that the inlet was located on a major incision of the bedrock. It can be inferred that the bedrock exerts control of channel location, this control varying in time as a function of channel depth. A conceptual model is proposed, including the inlet, its adjacent shorelines, the tidal bay and the time-varying bedrock control of main channel location. Such a model could be considered valid in similar cases along other coastlines, i.e. coastlines with a fine unconsolidated sediment sheet.

Archaeal Methane Cycling Communities Associated with Gassy Subsurface Sediments of Marennes-Oléron Bay (France)

In Marennes-Oléron Bay, a macro-tidal bay located on the French Atlantic coast, kilometer-scale acoustic turbidity reveals an accumulation of free gas in the sediment. Large concentrations of organic matter and rapid sedimentation rates provide ideal settings for biogenic methane cycling. We integrate seismic, sedimentologic, biogeochemical and molecular genetic approaches to determine whether microbial methane cycling is involved in this process. Here we show that the acoustic turbidity upper boundary matched with X-ray facies displaying fissures with the highest methane concentrations, demonstrating the existence of methane bubbles in the sediment. 16S rRNA and mcrA gene clone libraries were dominated by sequences affiliated to the three known ANME lineages and to putative methanogens. Sequences related to the marine benthic group B (MBG-B) and miscellaneous crenarchaeotal group (MCG) were also detected. However, the highest methane concentration facies was the only section where active Archaea were detected, using reverse-transcribed rRNA, indicating that these communities were involved either directly or indirectly in the methane cycling process. Moreover, three metabolically active novel uncultivated lineages, related to putative methane cycling Archaea, could be specifically associated to these methane bearing sediments. As methane cycling Archaea are commonly retrieved from deep subseafloor and methane seep sediment, the study of coastal gassy sediments, could therefore help to define the biogeochemical habitats of deep biosphere communities.

Climate control on late Holocene high-energy sedimentation along coasts of the northeastern Atlantic Ocean

Abundant sedimentological and geochronological data gathered on European sandy coasts highlight major phases of increased high-energy sedimentation in the North Atlantic Ocean during the late Holocene. Owing to an inconsistent use of the terminology, it is often difficult to determine whether studies have described storm-built or wave-built deposits. Both deposits can be identified by overall similar coarse-grained sedimentary facies, but may provide contradictory paleoenvironmental interpretations. The aim of this study is to address this issue, by analysing a set of published 14C ages recovered from wave-built sediment bodies of the Pertuis Charentais (France). Integration of 14C data highlights seven coarse-grained sedimentation pulses (CSPs) that reflect a synchronous increase in wave-induced sediment supply occurring around 2650, 2420, 1240, 970, 800, 650 and 450 cal. yr B.P. CSPs can be matched with geochronological data published on other exposed sandy coasts of the western Europe. In first order, CSPs were preserved during phases of decreased storm activity and thus limited coastal erosion in the North Atlantic Ocean from 2700 to 2000 and from 1200 to 600 cal. yr B.P. In second order, CSPs occurred during positive NAO and/or negative EA/WR peaks, which are known from previous studies to induce higher waves and to enhance wave-induced sediment transport in the Pertuis Charentais. Chronological boundaries of CSPs could be used as a diagnostic tool to differentiate storm deposits indicative of sudden, episodic very high-energy sedimentation from wave deposits resulting from the sustained action of above-average westerly winds blowing across the North Atlantic Ocean.