Jean-Luc Schneider

Dynamic disintegration processes accompanying transport of the Holocene Flims sturzstrom (Swiss Alps)

Abstract A sturzstrom is a large-scale landslide that evolves during transport into a rapid granular flow (rock avalanche) by a dynamic disintegration process. Numerous theoretical hypotheses have been proposed to explain the excessive mobility of these catastrophic mass movements, but the role of disintegration processes remains poorly understood. Exceptional outcrop conditions at the giant (12 km3) Flims sturzstrom (Swiss Alps) permit the analysis of disintegration processes that result from shearing during transport. In confined zones of the rock mass, shearing is concentrated along preferred surfaces (originating as bedding planes), which evolve into shear band layers. Differential velocities of slabs separated by these shear layers induce oblique fractures in the slabs by accommodation. Contacts between grains are maintained in the shear layers. In unconfined zones near the top and the lateral margins, shearing generates dilatancy. The rock mass is affected by anisotropic dispersion without mixing, and disintegrated material is preferentially produced to the top due to inflation. The rock slide then evolves to a granular flow. The consequence of these processes is the formation of two distinct depositional facies in the sturzstrom. The structured facies exhibits a stratified aspect close to the base, reflecting confined slab-on-slab shearing motion. Towards the top and distally, it evolves into a chaotic facies with some isolated pockets of the structured facies. Reduction of confining forces on the moving sturzstrom allows transformation of the structured facies into the chaotic facies. This characteristic transition, observable in many sturzstrom deposits, seems to be necessary for the rock avalanche process to occur, and offers insights into the long-runout characteristics of these catastrophic mass movements.

Primary or secondary distal volcaniclastic turbidites: how to make the distinction? An example from the Miocene of New Zealand (Mahia Peninsula, North Island)

Abstract Miocene marine volcaniclastic deposits occur in Mahia Peninsula (North Island, New Zealand) and were sedimented in a forearc setting related to the Hikurangi trench subduction system. These deposits are interbedded with hemipelagic marls, and correspond to simple or amalgamated centimetric- to metric-thick turbiditic sequences. Volcaniclastic material is mainly composed of vitric particles, with crystals (quartz, plagioclase with minor biotite, amphibole, pyroxene and oxides), which are well represented in the coarse-grained fraction. The glass shards are mainly rhyolitic in composition. Three types of volcaniclastic turbidites were distinguished with geochemical data, because distinction is impossible on sedimentary characteristics. (1) Primary monomagmatic turbidites contain both magmatic (bubble wall pumice and shards) and phreatomagmatic (blocky shards with few vesicles and hydroclastically fragmented pyroclasts) vitric particles. The chemical compositions of the vitric particles and the crystals are very homogeneous suggesting a cogenetic origin. These turbidites directly result from unique eruptive events and are probably related to the entrance of hot subaerial pyroclastic flows into the sea, which also led to their transformation into subaqueous gravity flows. (2) Secondary monomagmatic turbidites never contain phreatomagmatic pyroclasts and the glass compositions display a trend from andesites to rhyolites. There is a strong linear correlation in the compositions that suggest that the glass particles are derived from the same magma. Crystals also show a compositional homogeneity. These deposits reflect the succession of several eruptions related to a unique magmatic system and result of the reworking of volcaniclastic material after relatively short storage on the shelf. (3) Secondary multimagmatic turbidites do not display compositional homogeneity of their vitric and mineral components. This implies that the volcaniclastic material has been stored during a relative long period on the shelf before remobilization, and that this sedimentation records the volcanic activity of multiple magmatic sources. Consequently, it has been possible to distinguish primary volcaniclastic turbidites that are directly related to the volcanic activity, from secondary turbidites that result from reworking of previously deposited material on the shelf. Monomagmatic turbidites can be used as stratigraphic and magmatic markers whereas multimagmatic cannot. Multimagmatic turbidites, however, record the evolution of the volcanic arc during longer periods. This leads to the conclusion that the Mahia Peninsula volcaniclastic turbidites distally record the evolution of the source volcanic arc.

Chapter 7 - Volcaniclastic Processes and Deposits in the Deep-Sea

Abstract Volcanic activity at ocean-spreading centers, large oceanic islands, and especially subduction zones, leads to several types of eruptive and non-eruptive processes that generate large volumes of volcaniclastic sediments, and results in the deposition of fragmental material in the deep-sea. Volcaniclastic material forms an important part of sedimentary successions, particularly along active margins of both cordilleran and islandarc environments, and around large volcanic oceanic islands. This chapter presents an overview of the large diversity of volcaniclastic processes and deposits that can be recognized in deep-marine environments (i.e. beyond the shelf/slope break). The various aspects of their origin, recognition, and interpretation will be emphasized from the individual depositional interval to the volcaniclastic apron scale.

Two examples of subaqueously welded ash-flow tuffs: the Visean of southern Vosges (France) and the Upper Cretaceous of northern Anatolia (Turkey)

Abstract Pyroclastic deposits interpreted as subaqueous ash-flow tuff have been recognized within Archean to Recent marine and lacustrine sequences. Several authors proposed a high-temperature emplacement for some of these tuffs. However, the subaqueous welding of pyroclastic deposits remains controversial. The Visean marine volcaniclastic formations of southern Vosges (France) contain several layers of rhyolitic and rhyodacitic ash-flow tuff. These deposits include, from proximal to distal settings, breccia, lapilli and fine-ash tuff. The breccia and lapilli tuff are partly welded, as indicated by the presence of fiamme, fluidal and axiolitic structures. The lapilli tuff form idealized sections with a lower, coarse and welded unit and an upper, bedded and unwelded fine-ash tuff. Sedimentary structures suggest that the fine-ash tuff units were deposited by turbidity currents. Welded breccias, interbedded in a thick submarine volcanic complex, indicate the close proximity of the volcanic source. The lapilli and fine-ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales. Presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm-wave base. In northern Anatolia, thick massive sequences of rhyodacitic crystal tuff are interbedded with the Upper Cretaceous marine turbidites of the Mudurnu basin. Some of these tuffs are welded. As in southern Vosges, partial welding is attested by the presence of fiamme and fluidal structures. The latter are frequent in the fresh vitric matrix. These tuff units contain a high proportion of vitroclasis, and were emplaced by ash flows. Welded tuff units are associated with non-welded crystal tuff, and contain abundant bioclasts which indicate mixing with water during flowage. At the base, basaltic breccia beds are associated with micritic beds containing a marine fauna. The welded and non-welded tuff sequences are interbedded in an alternation of limestones and marls. These limestones are rich in pelagic microfossils. The evidence above strongly suggest that in both examples, tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment. The sources of the pyroclastic material are unknown in both cases. We propose that the ash flows were produced during submarine fissure eruptions. Such eruptions could produce non-turbulent flows which were insulated by a steam carapace before deposition and welding. The welded ash-flow tuff deposits of southern Vosges and northern Anatolia give strong evidence for existence of subaqueous welding.

The extreme mobility of debris avalanches: A new model of transport mechanism

Large rockslide-debris avalanches, resulting from flank collapses that shape volcanoes and mountains on Earth and other object of the solar system, are rapid and dangerous gravity-driven granular flows that travel abnormal distances. During the last 50 years, numerous physical models have been put forward to explain their extreme mobility. The principal models are based on fluidization, lubrication, or dynamic disintegration. However, these processes remain poorly constrained. To identify precisely the transport mechanisms during debris avalanches, we examined morphometric (fractal dimension and circularity), grain size, and exoscopic characteristics of the various types of particles (clasts and matrix) from volcanic debris avalanche deposits of La Reunion Island (Indian Ocean). From these data we demonstrate for the first time that syn-transport dynamic disintegration continuously operates with the increasing runout distance from the source down to a grinding limit of 500 µm. Below this limit, the particle size reduction exclusively results from their attrition by frictional interactions. Consequently, the exceptional mobility of debris avalanches may be explained by the combined effect of elastic energy release during the dynamic disintegration of the larger clasts and frictional reduction within the matrix due to interactions between the finer particles.

Gravity‐Driven Deposits in an Active Margin (Ionian Sea) Over the Last 330,000 Years

In the Ionian Sea, the subduction of the Nubia plate underneath the Eurasia plate leads to an important sediment remobilisation on the Calabrian Arc and the Mediterranean Ridge. These events are often associated with earthquakes and tsunamis. In this study, we analyse gravity-driven deposits in order to establish their recurrence time on the Calabrian Arc and the western Mediterranean Ridge. Four gravity cores collected on ridges and slope basins of accretionary prisms record turbidites, megaturbidites, slumping and micro-faults over the last 330,000 years. These turbidites were dated by correlation with a hemipelagic core with a multi-proxy approach: radiometric dating, δ18O, b* colour curve, sapropels and tephrochronology. The origin of the gravity-driven deposits was studied with a sedimentary approach: grain-size, lithology, thin section, geochemistry of volcanic glass. The results suggest three periods of presence/absence of gravity-driven deposits: a first on the western lobe of the Calabrian Arc between 330 000 and 250 000 years, a second between 120 000 years and present day on the eastern lobe of the Calabrian Arc and over the last 60 000 years on the western lobe, and a third on the Mediterranean Ridge over the last 37 000 years. Return times for gravity-driven deposits are around 1000 years during the most important record periods. The turbidite activity also highlights the presence of volcaniclastic turbidites that seems to be link to the Etna changing morphology over the last 320 000 years.