Antonio Cattaneo

The late-Holocene Gargano subaqueous delta, Adriatic shelf: Sediment pathways and supply fluctuations

The Gargano subaqueous delta formed on the eastern and southeastern sides of the Gargano promontory, in the western Adriatic. This subaqueous deposit represents the southernmost portion of the late-Holocene highstand systems tract (HST) growing along the western side of the Adriatic as an extensive wedge of deltaic and shallow-marine mud. The late-Holocene HST rests above a regional downlap surface that marks the time of maximum landward shift of the shoreline attained around 5.5 cal. kyr BP, at the end of the late-Pleistocene–Holocene sea-level rise. High-resolution seismic–stratigraphic and tephra correlation indicate the presence of a thin basal unit recording condensed deposition between 5.5 and 3.7 cal. kyr BP over much of the basin. Above this unit, sediment accumulation rates increased to high values (as much as 1.5 cm yr−1) reflecting the stabilisation of relative sea level and the forcing from high frequency climatic or anthropogenic changes affecting river dynamics. The late-Holocene mud wedge, of which the Gargano subaqueous delta is a significant component, reaches up to 35 m in thickness and has a volume of ca 180 km3. The shore-parallel thickness distribution of the mud wedge reflects the dominant oceanographic regime of the basin and the asymmetric location of the mostly western sediment sources (with a combined modern delivery of 51.7×106 t yr−1 of mean suspended load). In sections perpendicular to the coast the late-Holocene mud wedge appears composed of forestepping clinoforms with gently dipping foresets (typically 0.5°). The Gargano subaqueous delta is characterised by a submarine topset in water depths shallower than 25–28 m, and accounts for about 1/7th of the total volume of the late-Holocene mud wedge, despite the absence of direct river supply to the Gargano area. In the area of maximum interaction between shore-parallel currents and basin morphology, progradation occurs onto a flat and barren bedrock outcrop in about 50–80 m water depth. The rapid transition from a thickness of 30 m of late-Holocene mud to nil is a good indication of the role of southward-flowing bottom-hugging shelf currents in causing the redistribution of sediment along the Adriatic inner shelf. Additional evidence of this regime comes from: (1) the most recent sigmoid (defined at seismic–stratigraphic scale) deposited since the onset of the Little Ice Age, showing a shore-parallel thickness distribution and a main depocentre to the southeast of the Gargano promontory; (2) the maximum values of sediment accumulation rates over the last century (documented by 210Pb measurements) defining a narrow shore-parallel belt immediately seaward of the depocentre of the most recent sigmoid. The Gargano subaqueous delta grows from the outbuilding of progressively younger progradational sigmoids that tend to parallel the previous ones. The Gargano subaqueous delta differs from other documented late-Holocene subaqueous deltas because its growth reflects: (1) sediment transport dominated by bottom currents sub-parallel to the strike of the composing clinoforms; (2) a complex supply regime including the Po delta (350 km to the north) and several coalescing Apennine rivers acting as ‘line source’; (3) several alternating intervals of enhanced outbuilding and condensed deposition; and (4) an in-phase growth of the most recent sigmoid with the major progradation of the Po delta during the Little Ice Age.

Tectonic expression of an active slab tear from high-resolution seismic and bathymetric data offshore Sicily (Ionian Sea)

Subduction of a narrow slab of oceanic lithosphere beneath a tightly curved orogenic arc requires the presence of at least one lithospheric scale tear fault. While the Calabrian subduction beneath southern Italy is considered to be the type example of this geodynamic setting, the geometry, kinematics and surface expression of the associated lateral, slab tear fault offshore eastern Sicily remain controversial. Results from a new marine geophysical survey conducted in the Ionian Sea, using high-resolution bathymetry and seismic profiling reveal active faulting at the seafloor within a 140 km long, two-branched fault system near Alfeo Seamount. The previously unidentified 60 km long NW trending North Alfeo Fault system shows primarily strike-slip kinematics as indicated by the morphology and steep-dipping transpressional and transtensional faults. Available earthquake focal mechanisms indicate dextral strike-slip motion along this fault segment. The 80 km long SSE trending South Alfeo fault system is expressed by one or two steeply dipping normal faults, bounding the western side of a 500+ m thick, 5 km wide, elongate, syntectonic Plio-Quaternary sedimentary basin. Both branches of the fault system are mechanically capable of generating magnitude 6–7 earthquakes like those that struck eastern Sicily in 1169, 1542, and 1693.

Control of Quaternary sea‐level changes on gas seeps

Gas seeping to the seafloor through structures such as pockmarks may contribute significantly to the enrichment of atmospheric greenhouse gases and global warming. Gas seeps in the Gulf of Lions, Western Mediterranean, are cyclical, and pockmark “life” is governed both by sediment accumulation on the continental margin and Quaternary climate changes. Three-dimensional seismic data, correlated to multi-proxy analysis of a deep borehole, have shown that these pockmarks are associated with oblique chimneys. The prograding chimney geometry demonstrates the syn-sedimentary and long-lasting functioning of the gas seeps. Gas chimneys have reworked chronologically constrained stratigraphic units and have functioned episodically, with maximum activity around sea level lowstands. Therefore, we argue that one of the main driving mechanisms responsible for their formation is the variation in hydrostatic pressure driven by relative sea level changes.

Holocene turbidites record earthquake supercycles at a slow-rate plate boundary

Ongoing evidence for earthquake clustering calls for records of numerous earthquake cycles to improve seismic hazard assessment, especially where recurrence times overstep historical records. We show that most turbidites emplaced at the Africa-Eurasia plate boundary off west Algeria over the past ∼8 k.y. correlate across sites fed by independent sedimentary sources, requiring a regional trigger. Correlation with paleoseismic data inland and ground motion predictions support that M ∼7 earthquakes have triggered the turbidites. The bimodal distribution of paleo-events supports the concepts of earthquake supercycles and rupture synchronization between nearby faults: 13 paleo-earthquakes underpin clusters of 3–6 events with recurrence intervals of ∼300–600 yr, separated by periods of quiescence of ∼1.6 k.y. without major events on other faults over the study area. This implies broad phases of strain loading alternating with phases of strain release. Our results suggest that fault slip rates are time dependent and call for revising conventional seismic hazard models.

Comment on “Zemmouri earthquake rupture zone (Mw 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model” by A. Ayadi et al.

Although often difficult to characterize, the relationship between a seismic rupture, its aftershock sequence, and cumulative subsurface or surface faulting or folding is an important challenge to modern seismology and seismotectonics. Among other benefits, it helps document fault length, slip, and magnitude relationships, reconstruct the evolution of the rupture process through historical and prehistorical times and identify the complexity of the deformation in its path toward the surface. This approach is a prerequisite to any seismic hazard assessment but is particularly difficult for faults whose surface trace projects offshore. A specific effort to identify and quantify the source parameters of large earthquakes in coastal areas is therefore needed, not only in subduction zones but also in areas of slow rate and/or diffuse deformation.

Submarine Landslides Along the Algerian Margin: A Review of Their Occurrence and Potential Link with Tectonic Structures

The Algerian continental margin, a Cenozoic passive margin along the plate boundary between Eurasia and Africa presently reactivated in compression, is one of the most seismically active areas in the Western Mediterranean, having experienced several moderate to strong earthquakes in the coastal zone during the last century. The morphology of the continental slope offshore Algeria is steep and dominated by the presence of numerous canyons of variable size and sea-floor escarpments that are probably the seafloor expression of active thrust-folds. Numerous submarine landslides are present along these structures, as well as asso-ciated with salt diapirs in the abyssal plain. Submarine landslides are expressed by seafloor scars (usually of small size) and subsurface or buried acoustically chaotic/ transparent units interpreted as mass transport deposits (MTD). Compared with the most recent (superficial) small size of slide scars, buried MTDs seem to be of larger size, possibly suggesting a change in the functioning of gravity-driven processes in the margin throughout the Plio-Quaternary.

Palaeomagnetic and rock magnetic analysis of Holocene deposits from the Adriatic Sea: detecting and dating short-term fluctuations in sediment supply

We present palaeomagnetic and rock magnetic results from five cores collected in the basin of the Adriatic Sea. Four cores (PRAD2-4; KS02-246; CSS00-23 and CSS00-07) were collected along a shore-parallel transect whereas the fifth core (AMC99-01) was retrieved in the central Adriatic in 250 m water depth, on the floor of the Meso-Adriatic Depression. After alternating field demagnetization, the natural remanent magnetization (NRM) directions show a characteristic and primary magnetization of the sediments that is representative of secular variation of the geomagnetic field. By combining 14C dating and the ages of magnetic inclination features, it has been possible to establish an age—depth model for all of the cores. Precise identification of the secular variation features provides a refined chronology that allows us to quantify short-term changes in sediment flux from the Adriatic catchment as well as changes in sediment routing within the basin. Rock magnetic parameters indicate a homogeneous magnetic mineralogy dominated by fine-grained magnetite. Variations in both concentration and grain-size related magnetic parameters reflect changes in sediment supply between glacial and interglacial stages, as well as changes in environmental conditions. Selective dissolution of magnetic grains is associated with the formation of sapropel S1 during the first part of the Holocene, although the duration of the anoxic interval varies between cores. A significant modification in sediment supply characterizes the late-Holocene deposits with a marked increase in sedimentation rate occurring during the last few centuries. We interpret this as a far-reaching response to the rapid construction of the modern Po delta under increasing anthropogenic impact.

Failure Processes and Gravity-Flow Transformation Revealed by High-Resolution AUV Swath Bathymetry on the Nice Continental Slope (Ligurian Sea)

The continental slope offshore Nice is a natural laboratory to investigate submarine landslides and gravity-flow processes. Using EM300 bathymetry data (spatial resolution of 25 m), about 250 scars with volume less than 8 × 108 m3 were identified. The AUV bathymetric data (spatial resolution of 2 m) revealed a greater number of scar-related failures with two main morphologies: some scars are affected by retrogressive processes of erosion, suggesting failures were triggered a long time ago, while other scars exhibit no evidence of post-failure erosion, suggesting they could have been triggered recently. Downslope from the scars, there are scattered blocks, on average 5-m high and 40-m wide, and well-developed asymmetrical waves, on average 2-m high and 20-m in wavelength. Such evolution could be evidence for the transformation of the remobilized deposits into cohesive flows then turbulent flows. Such transformation took place over a distance of less than 6–8 km.

Identification of Shear Zones and Their Causal Mechanisms Using a Combination of Cone Penetration Tests and Seismic Data in the Eastern Niger Delta

In a site investigation of the eastern part of the offshore Niger delta, cone penetration tests (CPTU) showed significant drops in tip resistance, associated with decreases in sleeve friction and induced excess pore pressures at the interface between superficial sediments and the underlying deposits of a mass-transport complex (MTC) called NG1. Such signature characteristics of weakened zones are clearly expressed at three sites where the drop in tip resistance reaches more than 40% over 2–3 m-thick intervals. Correlations between CPTU profiles and both 3D and ultrahigh-resolution 2D seismic data suggest that the weakened zones surround syndepositional the within the frontal part of NG1. Hence, weakening appears associated with the remobilization of thrust faults, inducing localized plastic shear. Relatively recent, deep-seated structural movements affecting NG1 are suspected to have remobilized these thrusts faults. When considering the sole influence of gravity, the fact that shear strength is mobilized within scattered, limited zones along steeply dipping syndepositional faults is not favorable for the further development of a continuous slope-parallel failure surface above NG1.

Detailed Analysis of a Submarine Landslide (SAR-27) in the Deep Basin Offshore Algiers (Western Mediterranean)

On May 21, 2003 an earthquake with a magnitude of 6.8 (Mw) struck the city of Boumerdes, located on the coast near Algiers, and generated significant gravity flows recognized by numerous submarine cable breaks. In order to investigate a possible offshore imprint of past earthquakes in terms of sediment instabilities, we analyzed one of the numerous escarpments of tectonic origin existing on the Algerian margin. This escarpment, located about 50 km NE offshore Algiers is within water depths between 2,600–2,900 m and has an average slope of 4°. A variety of geotechnical (piezocone), geophysical (swath bathymetry, Chirp echo-sounder and high-resolution deep-towed side scan sonar) and sedimentological (sediment cores) techniques were used to realize this work. Our study focuses on a well-defined submarine landslide scar (called SAR27) revealed by side-scan sonar images. The SAR 27 slide is small in size (0.5 × 1.7 km), but exhibits a complex morphology including a 10-m high headwall scar, tilted blocks, an area of reworked sediment and a distal deposit. Correlations between in-situ measurements, sampling and CHIRP profile provided a comprehensive understanding of the slide geometry and emphasized the role of thin silty sand beds in the slide initiation. These silty sand beds are probably acting as liquefied slip surfaces, implying a translational displacement mode which correlates the slide morphology.