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Geological complexity, progressive tectonic overprint and prevailing submarine location of subduction margins generally hinder investigating the early stages of development of their oldest trench-slope basins, witnesses of subduction initiation and early history. Along the eastern North Island of New Zealand, the inner portion of the Hikurangi subduction wedge is emerged, thereby offering a unique opportunity to examine, at outcrop-scale, the tectonostratigraphic evolution of a trench-slope basin (Castlepoint trench-slope basin) that was linked to the onset of subduction. We present new occurrences of lowermost Miocene gravity-driven systems, suggesting a more intricate depositional framework to the ones previously inferred during this key period. Results show that the early stages of development of a trench-slope basin coeval with the birth of a subduction margin may deviate from traditional models either comprising (1) sustainable sediment sources connected to the basin very early in the history of the margin and (2) a sedimentation-deformation feedback mechanism promoting the long-term development of an aggradational turbidite system downslope. Analysis of the mass-transport deposits (MTDs) also revealed that the outboard migration of deformation was discontinuous and uneven along the margin during the earliest Miocene. Two major tectonic events, separated by a period of reduced tectonic activity, were recorded at the inboard border of the Castlepoint trench-slope basin, each resulting in seaward motion, oversteepening and frontal denudation of a thrust sheet. Each thrust sheet provided contrasting failed material and morphometric characteristics to the associated deposits, thereby allowing us to discriminate the nature of the nappe and related controls responsible for shedding each MTD as well as refine the timing of nappe emplacement along this part of the margin. Overall, this study draws new insights on the early structural evolution and stratigraphic infill during the birth of subduction zones, insights which may, in turn, help improve understandings of active margin settings. • Thrust fault growth can result in a vertical partitioning of mass-wasting processes at its front (mass movement then flow). • MTDs can record denudation of their source by progressively incorporating clasts in reverse stratigraphic order. • Overpressure at the sole of nappes may favor preservation of underthrust deposits preventing significant tectonic reworking. • Early stages of trench-slope basin fill coeval with the birth of a subduction margin can deviate from classical models. • Outboard migration of deformation during the earliest Miocene was discontinuous and uneven along the Hikurangi Margin. [ABSTRACT FROM AUTHOR]

In order to better constrain the relationship between tectonics and sedimentation of the Eastern Sardinian Margin, from the Messinian to the present day (METYSS project), a lithostratigraphic study was carried out on the Late Neogene deposits along the Cedrino Valley (eastern Sardinia). The late Neogene evolution is documented from sedimentological and tectonic analysis, biostratigraphic dating, and by mapping and logging stratigraphic sections. Previous authors have differentiated an early Pliocene marine formation from an older continental formation, possibly Miocene, the entire system being covered by late Pliocene to Quaternary lava flows. Preliminary biostratigraphic data suggest the absence of the lowermost Pliocene, based on the absence of Globorotalia margaritae and the presence of Globorotalia puncticulata , implying a sediment age between 3.81 and 2.41 Ma. At the foot of the Fiuli cliff, some mud clasts include a coccolith microflora, Tortonian in age, suggesting reworking. The Pliocene marine deposits consist of a single filling sequence instead of a succession of superimposed sequences as we would expect during this period of time; the occurrences of terms rarely respect the usual transgressive-regressive succession. Contrasting bathymetric environments (fore-shore, pro-delta, shelf or upper slope) can be abruptly superimposed on top of the other, along few meter thick intervals. A rapidly flooded term is observed in several sites and indicates a spatial variability over only a few kilometers distance. Some deep water deposits are in the middle of the infilling Pliocene sequence, whereas shallower water deposits can be repeated along a vertical section, or even interrupted by emersion. These marked differences can express syn-, or possibly post-sedimentation tectonic uplifts. The 10–25° bed dips originated through local tectonic uplift. At Fiuli, the geometry of the bedding planes is indicative of tilting toward the west, in a direction opposite to the one observed at Onifai. These Pliocene series form a wide syncline with an approximately NNE–SSW axis. This morpho-structural behavior appears to be largely atypical of this East-Sardinian coastal area. [ABSTRACT FROM AUTHOR]

In the Hikurangi subduction wedge (New Zealand), a strong relationship exists between tectonic structures and fluid migrations. In the study area, outcropping tubular carbonate concretions, corresponding to the shallow subsurface plumbing systems of paleo-cold seeps, are hosted by Miocene syn-subduction mudstones. New observations demonstrate the presence of migrated solid bitumen within the tubular concretions and in the fault gouge of a major fault zone. A multi-proxy approach was performed to determine the organic matter thermal maturity in the study area (organic matter petrography and solid bitumen reflectance (BR) R r % (R r : random reflectance)). We also used Rock-Eval pyrolysis, vitrinite reflectance (VR) R r %, and clay mineral reaction progress (illite Kübler-Index and clay mineral paragenesis) to determine the diagenesis grade of the rocks. Low T max values and clay minerals indicate a thermally immature sedimentary cover. The main source rock of the region, the Waipawa Formation is locally thermally mature (VR = 0.86 R r %) suggesting that tectonic thrust-sheet stacking isresponsible for a structural thickening causing local organic maturation. The seaward propagation of out-of-sequence thrusts at base of intra-slope basins could be responsible for the inititation of biogenic fluid flows sourced in the shallow sedimentary cover that is subjected to deformation above the blind thrusts, leading to the earlier generation of the first carbonate tubular concretions. With the continuation of blind thrusting, deep thermogenic fluids then migrated laterally through fault planes (primary migration) and finally vertically through the intrabasinal pre-existing tubular concretions (secondary migration). In this paper, solid bitumen is used for the identification of a fossil thermogenic fluid migration from the source rock, along faults and through tubular carbonate conduits within a subduction thrust-wedge. The study evidences a multi-genetic tubular concretion formation, related with the timing and style of the deformation, being therefore a potential reliable indicator for the evolution of tectonic activity. • Integrated analysis of a plumbing system of paleo-cold seeps on an active margin. • Samples from SR, faults and tubular concretions to determine thermal maturity, bitumen migration, clay mineral reactions. • Circulation of thermogenic fluids in cold seep systems, which correspond initially to migration of shallow biogenic gas. • Role of the shallow plumbing system of biogenic cold seeps as vertical paths for subsequent thermogenic fluid circulations. • Structural thickening due to thrust sheet stacking favor maturation, thrust faults then acting as lateral migration pathway. [ABSTRACT FROM AUTHOR]

Along active margins, the combination of predominant tectonic activity and shallow-marine mixed siliciclastic-carbonate source systems developing upon and around actively growing structures challenges traditional source-to-sink models. This study aims to investigate the implications of mixed siliciclastic-carbonate shelfal domains located in contrasting geotectonic settings (thrust forelimb and backlimb) for the development of the concomitant gravity-driven systems beyond the shelf edges. Here, we document the vertical and lateral stratigraphic variabilities of the shelf-derived turbidites and mass-transport deposits (MTDs) at outcrop-scale through the integrated interpretation of photogrammetry, field and taphonomic data from the emerged southern portion of the Hikurangi subduction margin. Results highlight the role and importance of varying structural setting of the sediment source, whereby the different morphologies of the source regions (continent-attached forelimb, continent-detached backlimb) control the development of highly varied shelf-derived gravity-driven depositional systems that interact with the structures across the same confined intra-slope basin. The deposits are tens to a few hundred of meters in thickness and have a lateral extent of several kilometers. The depositional systems are characterized by durations of 1 to 2 Ma and were primarily controlled by the geometries and tectonic motion of the underlying structures at the shelf edges. Shelf-derived mass-wasting systems occurred on both sides of the actively growing thrust structures and were sourced from both shelfal domains that were attached or detached from the continental domain. When sourced from the backlimbs however, the subsequent MTDs exhibit more complex internal architectures, ultimately recording the dynamic changes in slope gradient, and can therefore be used as proxies for unraveling the tectonic activity of an individual structure. Our study provides new insights to better predict mixed siliciclastic-carbonate depositional settings along active margins, sourced from thrust forelimb and backlimb. These results may be important for deep-marine exploration and tectonostratigraphic reconstruction of fold-and-thrust belts. • Geometry and tectonic motion of the underlying structures at the shelf edges chiefly control mixed gravity-driven systems. • Shelf-derived mass-wasting events can occur on both sides of actively growing asymmetrical structures (thrusts). • MTDs can either be sourced from shelfal domains that are attached to the main continental domain or detached from it. • MTDs from thrust backlimb can exhibit more complex architectures ultimately recording the dynamic changes in slope gradient. • MTDs can be used as proxies for unraveling the tectonic activity of an individual structure [ABSTRACT FROM AUTHOR]

In Ibiza Island, we show for the first time that the post-tectonic late Miocene deposits are locally deformed in synsedimentary large scale boudinage structures. The late Miocene series is composed of a carbonate rich Unit 1 that is overlaid by a clastic Unit 2 and a Unit 3 in which a basal oolite-rich deposit is distinguished from an upper stromatolite-rich part. The oolite-rich and competent layers of Unit 3 are dissected into large rectangular to barrel-shape boudins that indicate a multidirectional extension. The separation of the blocks develops significantly northward revealing a stretching increase in a seaward direction that could have been favored by sliding of the distal part of the underlying marly unit over the clays. Along and in between the boudins, the laminated layers form soft-sediment syncline folds. The progressive bending of these laminated layers created spaces that were filled with calcite crystal wedges, providing evidence for large fluid circulation during deformation. Microthermometry on fluid inclusions in these calcite crystals revealed growth-temperatures ranging from 190 to 260 °C. This high recrystallization rate is an indication of important fluid circulation during deformation processes. Besides, the progressive infill of paleosols over some of the synsedimentary synclines indicates that deformation was a slow process during the emersion of the series. And the subaerial Messinian channels location in the topographic depressions formed by the boudinage of the late Miocene series confirms that the deformation and sliding occurred during the relative emersion associated with the Messinian Salinity Crisis. We thus interpret the boudinage as resulting from the destabilization of a still soft carbonate platform whose sliding has been helped substantially by the injection of relatively hot fluids. This work provides an onland evidence for extensional collapse during the Messinian sea-level drawdown. • We show synsedimentary large scale boudinage in the Late Miocene of NE Ibiza Island. • The badly organized chocolate tablet structure reveals a multidirectional extension. • The soft sediment deformation developed during slow translational sliding of boudins. • This sliding is due to margin collapse during the Messinian sea-level drawdown. [ABSTRACT FROM AUTHOR]

The METYSS project ( M essinian E vent in the T yrrhenian from S eismic S tudy) is based on high-resolution seismic data acquired along the Eastern Sardinian margin, Western Tyrrhenian Sea. The main aim is to study the Messinian Salinity Crisis (MSC) in the Western Tyrrhenian Basin, but we also investigated the thinning processes of the continental crust and the timing of crustal vertical movements across this backarc domain. Our first results shown that rifting ended before the MSC, but that crustal activity persisted long after the end of the rifting. This has been particularly observed on the proximal margin, the East-Sardinia Basin, where the Mobile Unit (MU, mobile Messinian salt) is thin or absent. In this study, we examined the distal margin, the Cornaglia Terrace, where the MU accumulated during the MSC and acted as a décollement , thus potentially decoupling the basement from the sedimentary cover. Our observations provide evidence for lateral flow and gravity gliding of the salt and its brittle sedimentary overburden along local basement slopes generated by the post-MSC tilting of some basement blocks formerly generated during the rifting. We also investigated an intriguing wedge-shaped body of MU located in a narrow N-S half graben bounded to the west by a major, east-dipping, crustal normal fault. Classically, one could think that this salt wedge is related to the syn-tectonics deposition of the MU, but we propose an original scenario, in which the post-rift vertical motion of the major fault has been cushioned by lateral flow of an initially tabular salt layer, leaving the supra-salt series apparently unaffected by the crustal motions of the basement. We tested this scenario by comparing natural data and physical (analogue) modelling data. Our results reveal that salt tectonics provides a powerful tool to understand the deep crustal tectonics of the margin and to constrain the timing of vertical motions in the Western Tyrrhenian Basin, results that can be applied to rifted salt-bearing margins worldwide. [ABSTRACT FROM AUTHOR]

In order to better constrain the chronology of the Messinian Salinity Crisis (MSC) onland and to provide new observations on the stratigraphic position of the Messinian erosional surface as well as evidence for continentalization, we have studied the Mio-Pleistocene succession of the island of Ibiza, ideally located in a strategic position between the peripheral Messinian basins of south-eastern Spain and the MSC-related gypsum of the Palma Basin (Mallorca). Five lithological units are identified and compared with those known from the island of Mallorca. The equivalent of the "reef" unit, mainly Tortonian in age, is composed of median to inner ramp facies. It is overlain by coastal alluvial fans set up under extensive tectonic control, and then by a carbonate unit rich in oolites and microbialites attributed to the Terminal Carbonate Complex (TCC) of Messinian age. The overlying red clays fossilizing paleosols express a continentalization phase at the end of the Miocene. This regressive episode is evidenced by karstification and erosion incising the pre-crisis units and shaping valleys connected to submarine canyons. The erosion surface is linked to the acme of the MSC, a major sea-level fall, and is then sealed by Pliocene coastal calcarenites or by Pleistocene eolianites. The emptying of the basin led to a global collapse, which is recorded on all the slope domains of the Mediterranean margins. We document this collapse for the first time onland by syn-MSC gravity events such as mudflows, upwelling of geothermal fluids under pressure and soft deformation of the TCC. The interweaving of these phenomena of multiple origin (baselevel decrease, climate change, geothermal activity, sediment transfer) contributes to the onland/offshore reshaping of the Mediterranean margins during the major environmental changes of the Messinian salinity crisis. • Two main regressions and transgressions control Upper Miocene sedimentation. • Extensional tectonic influence is well expressed during Tortono-Messinian interval. • The major regression with generalized emersion is recorded after TCC deposits. • The emersion is contemporaneous with gravitational instabilities and major erosion. • Major erosion is related to the MSC Margin Erosion Surface. [ABSTRACT FROM AUTHOR]

The current interpretation of the Messinian Salinity Crisis (MSC) involves the deposition of peripheral or marginal evaporites in onshore basins as well as the erosion of the margin and the deposition of thick evaporites in deep basins. The so-called intermediate basins are formed in domains between the onland outcrops and the deep basins. The Balearic Promontory is a bathymetric high located between the deep Algerian and Liguro-Provençal basins and the onland Spanish basin. The SIMBAD project aims to investigate the spatial variability of the MSC-related deposits and to assess the extent of post-MSC reactivation over the Balearic Promontory. We present here the first results of the SIMBAD high-resolution seismic survey (January 2013) which imaged for the first time a thin MSC-related unit widely distributed in small sub-basins over the Balearic Promontory. Borehole analyses have shown that this unit could be correlated with primary gypsum formations linked to the peripheral evaporites. Locally, in the Central Depression between Mallorca and Ibiza islands, a thicker MSC unit is observed whose lowermost transparent part could correspond to a salt layer. Geometrical relationships suggest that the MSC in the Central Depression could postdate the primary gypsum. The occurrence of a halite layer in the Central Depression, at depths of 1000 to 1500 m, favours the hypothesis that the evaporites precipitated passively in closed or partially closed perched sub-basins, possibly as a result of evaporative drawdown at different depths and possibly diachronously, at least with respect to the deep-basin evaporites. [ABSTRACT FROM AUTHOR]