Rheological Transitions During Partial Melting and Crystallization with Application to Felsic Magma Segregation and Transfer

We consider the rheological behaviour of felsic magma in thezone of partial melting and during subsequent crystallization.We also introduce and combine concepts (mushy zone, percolationtheory, granular flow, shear localization) derived from thenon-geological literature and apply them to field observationson migmatites and granites. Segregation and transportation offelsic magmas is commonly observed in association with non-coaxialdeformation, suggesting that gravity forces have limited influenceduring magma segregation. Solid to liquid and liquid to solidtransitions are shown to be rheologically different, which infirmsthe concept of a unique rheological critical melt percentagefor both transitions. Four stages are examined, which dependon the melt fraction present. (1) A minimum of 8% melt by volume must first be produced toovercome the liquid percolation threshold (LPT) above whichmelt pockets can connect, thus allowing local magma displacement.Transport of the liquid phase is amplified by deformation towarddilatant sinks and is restricted to a very local scale. Thiscorresponds to partially molten domains illustrated by incipientmigmatites. (2) When more melt (20–25%) is present, a melt escapethreshold (MET) allows segregation and transport of the meltand part of the residual solid phase, over large distances.This corresponds to segregation and transfer of magma towardsthe upper crust. (3) Segregation of magma also occurs during granite emplacementand crystallization. In a flowing magma containing few particles(20%), particles rotate independently within the flow, defininga fabric. As soon as sufficient crystals are formed, they interactto construct a rigid skeleton. Such a random loose packed frameworkinvolves 55% solids and corresponds to the rigid percolationthreshold (RPT). Above the RPT, clusters of particles can sustainstress, and the liquid fraction can still flow. The only remainingpossibilities for rearranging particles are local shear zones,often within the intrusion rim, which, as a consequence, developsdilatancy. This stage of segregation during crystallizationis totally different from that of magma segregation during incipientmelting. (4) Finally, the system becomes totally locked when random closepacking is reached, at 72–75% solidification; this isthe particle locking threshold (PLT). The introduction of four thresholds must be viewed in the contextof a two-fold division of the cycle that generates igneous rocks,first involving a transition from solid to liquid (i.e. partialmelting) and then a transition from liquid to solid (i.e. crystallization).Neither transition is simply the reverse of the other. In thecase of melting, pockets of melt have to be connected to afforda path to escaping magma. This is a bond-percolation, in thesense of physical percoloation theory. In the case of crystallization,randomly distributed solid particles mechanically interact,and contacts between them can propagate forces. Building a crystalframework is a site-percolation, for which the threshold ishigher than that of bond-percolation. For each transition twothresholds are applicable. The present approach, which basicallydiffers from that based on a unique critical melt fraction,expands and clarifies the idea of a first and a second percolationthreshold. One threshold in each transition (LPT and RPT, respectively)corresponds to a percolation threshold in the sense of physicalpercolation theory. Its value is independent of external forces,but relies on the type and abundance of minerals forming thematrix within which melt connectivity is developing. The exactvalue of the second threshold (MET or PLT) will vary accordingto external forces, such as deformation and the particle shape. KEY WORDS: migmatites; partial melting; granites; magma segregation; magma solidification ^*Corresponding author. Telephone: 33 03 83 44 19 00. Fax: 33 03 83 44 00 29. e-mail: jlv{at}cregu.cnrs-nancy.fr     

From non‐tidal shelf to tide‐dominated strait: The Miocene Bonifacio Basin,Southern Corsica

This study documents a change from a non‐tidal to tide‐dominated shelf system that occurred between Corsica and Sardinia (the Bonifacio Basin, Western Mediterranean) during the early to middle Miocene. The non‐tidal deposits formed on a low‐energy siliciclastic shelf surrounded by progradational coralline algal ramps at full highstand. The tidal deposits consist of an up to 200 m thick succession of siliciclastic to coralline‐rich cross‐beds formed by large sub‐tidal dunes. Based on outcrop and sub‐surface data, it is possible to conclude that the tidal currents were amplified as a consequence of the rapid subsidence of the basin centre due to tectonic activity. It is suggested that this tectonic event initiated the strait between Corsica and Sardinia. The strait was deep enough to allow the tidal flux to be significantly increased, generating a localized strong tidal current at the junction between the Western Mediterranean and the East Corsica Basin.     

Late Neanderthal occupation in North‐West Europe: rediscovery,investigation and dating of a last glacial sediment sequence at the site of La Cotte de Saint Brelade,Jersey

In 2011, a programme of field research was undertaken to effect the stabilization of an unstable section in the West Ravine at the key Neanderthal occupation site of La Cotte de St Brelade on the Channel Island of Jersey. As part of this essential remedial work the threatened section was analysed to characterize its archaeological and palaeoenvironmental potential as well provide optically stimulated luminescence (OSL) dates. The work determined, through two concordant OSL dating programmes, that the section formed part of an extensive sequence of sedimentation spanning >105 to <48 ka. Furthermore, reanalysis of the archive determined that the sediment sequence examined contained the stratigraphic equivalent of deposits lying below those that have previously produced Neanderthal fossils. Through our work, we can now constrain these younger sediments to being younger than 48 ka. The combined results suggest that this sequence now represents the recovery of an extensive dataset, thought lost to science through complete excavation, which holds the potential to throw light on the disappearance of Neanderthal populations from the Atlantic‐edge outpost on the north‐west frontier of their world.     

Evolution of the Nile deep‐sea turbidite system during the Late Quaternary: influence of climate change on fan sedimentation

This paper presents an overview of the evolution of the Nile deep‐sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep‐sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large‐scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum (ca 25 to 14·8 ka) the central and eastern parts of the Nile deep‐sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea‐level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood‐derived deposits occurred across the shallower part of the submarine delta during sea‐level rise between ca 14·8 and 5 ka. The most recent deep‐sea channel–lobe system was very active during this period of rising sea‐level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large‐scale slope failure. The Nile deep‐sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep‐sea turbidite system may be more active during periods of rising and high sea‐level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea‐level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large‐scale sedimentary system responded to monsoon‐driven climate change and sea‐level change in a system‐wide and contemporaneous manner.     

The Watten boring - an Early Weichselian and Holocene climatic and palaeoecological record from the French North Sea coastal plain

A deep boring (30 m), carried out at Watten (Nord, France) in 1990, has provided an opportunity to undertake palaeoecological studies based on stratigraphy, sedimentology, palynology, malacology and ¹⁴C datings on Early Weichselian and Holocene deposits. The location of the site in the inner zone of the coastal plain, extending in the Aa valley from the southernmost shores of the North Sea upstream to Watten, allows the comparison between interstadials recorded in a pure fluvial environment and the overlying Holocene sediments, in which alternation of marshy, fluvial and marine deposits are observed. The Early Weichselian sequence can be correlated with the Brørup and Odderade interstadials described in northwestern Europe, and belongs within the same palaeogeographical area. At that time the conditions were already continental at Watten. However, the last (Holocene) interglacial has been characterized, at least since the Boreal chronozone, by an oceanic climate and a progressive invasion of the sea, sometimes interrupted by stillstands and withdrawals.     

VLF RESISTIVITY MAPPING AND VERTICALIZATION OF THE ELECTRIC FIELD1

For over 20 years, powerful VLF transmitters have been used as electromagnetic sources for subsurface investigations in mining exploration. Measurements initially concerned the vertical component of the magnetic field or the inclination of the field and were later extended to measurement of the horizontal electric field in the direction of the transmitter, to determine the resistivity of the terrain. Measurement of the electric field is usually performed with electric lines, grounded or not, with lengths of at least 5 m. This paper presents the concept of a VLF resistivity meter with a very short electric sensor (1 m) and the results obtained with it. This technique improves the measurement of the electric field, which is in principle a point value. It also permits a higher spatial sampling rate and, by closely linking the electric sensor with the magnetic sensor on a lightweight mount, makes it possible for the instrument to be used by a single operator. In addition, transformation of the electric field data, analogous to reduction to the pole in magnetism, is proposed to correct the horizontal deformation of the anomalies created by polarization of the primary field. Comparison with direct current electrical measurements shows highly satisfactory correlations. This transformation, validated for VLF, can be extended to any electrical or electromagnetic method using a uniform primary field, i.e. gradient array in direct current or low-frequency magnetotellurics. We call this verticalization of the electric field. Resistivity measurements and mapping using the VLF frequency range can be applied not only to mining but also to a wide range of shallow geophysical studies (hydrology, civil engineering, etc.) and are not limited to problems concerning the location of conductive targets     

Rudist-coral frameworks associated with submarine volcanism in the Maastrichtian of the Pachino area (Sicily)

ABSTRACT
Maastrichtian strata from the Pachino area (SE Sicily) provide a model of association between rudist-coral frameworks and submarine volcanic activity.
Two successive carbonate units are distinguished: (a) Coral-rudist bioherms and biostromes developed in a high-energy environment, (b) Hippuritid build-ups and banks overlain by rudist-coral clusters which grew under harsher ecological conditions, in a weaker current regime.
At the top of the Maastrichtian sequence, the growth of rudist-coral frameworks ceased as ecological conditions shifted toward restricted environments. The inferred succession reflects a gradual decrease in current strengths and water depths, related to the Maastrichtian regressive phase.
The frameworks display consistent evolutionary sequences reflecting progressive changes in their biota, structure, size and shape.
Occurrences of rudist-coral frameworks are clearly linked with submarine volcanoes which provided opportunities for their development; they sometimes grew in the vicinity of active-vent centres, a feature reported for the first time in the Upper Cretaceous.     

Turbiditic,clay‐rich event beds in fjord‐marine deposits caused by landslides in emerging clay deposits – palaeoenvironmental interpretation and role for submarine mass‐wasting

Distinct, clay‐rich beds are common in fjord‐marine deposits in Trondheimsfjorden near the outlet of the Nidelva River. Their characteristic light‐grey colour makes the beds easily distinguishable from the surrounding brownish, bioturbated, muddy fjord sediments. The clay‐rich beds commonly display a clear stratification in clay, silt and very fine sand. The beds are interpreted as originating primarily from large quick‐clay landslides upstream along the Nidelva River. Such events resulted in a sudden increase in the supply of fines to the fjord from disintegrating landslide debris and heavily loaded effluent plumes, possibly favouring hyperpycnal flow. Typical beds can be divided into a clay‐rich lower section, reflecting an initial surge with high concentrations of suspended mud, and a sandier upper section reflecting pulses of higher energy. This development can be explained, for example, by a lowering in the supply of mud, an increasing activity of deltaic sediment gravity flows due to a higher availability of sandy sediments in the landslide‐affected river, and by flooding and/or breaching of landslide dams. The typical, stratified beds are interpreted as the result of one quick‐clay landslide, whereas exceptionally thick, less organized, stratified beds are possibly the result of several large and/or complex landslides. Radiocarbon dating of mollusc shells has helped to establish a chronology for major terrestrial landslides in the area. The frequency of landslides increases towards the end of the Holocene. This is explained by a progressively deeper incision of rivers during glacioisostatic rebound, possibly combined with a change to a wetter climate. The marine core record displays deformation structures and hiati representing submarine mass‐wasting events, and supports the evidence that the clay‐rich beds are weak layers in the fjord‐marine stratigraphy. The inherent weakness of these layers may be explained by their composition, immature texture, loose fabric and contrasting permeabilities in the deposits. Slide‐prone layers similar to the clay‐rich beds described here may be found in other comparable fjord‐marginal settings and are considered to be of importance for geohazard assessments.     

Extensive carbonate algal bioherms in upper Pleistocene saline lakes of the central Altiplano of Bolivia

During the upper Pleistocene the Central Altiplano of Bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations. Development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands, discontinuously covering the 300-km-long and 100-km-wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of Uyuni. Distribution, size and shape of the bioherms are diverse due to various factors, e.g. the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth. On larger palaeoterraces, the build-ups coalesced to form platform-like carbonate accumulations. Although the morphologies closely resemble those induced by cyanobacteria, they were predominantly constructed by other plant communities, probably dominated by filamentous green algae. Cyanobacterial communities flourished in association with these plants, but they did not contribute significantly to the architecture of the bioherms; they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build-ups. A prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects. The build-ups located between 3660 and 3680 m elevation display up to three major parts: (1) a massive inner core formed during an early Minchin highstand, before 40 ka; (2) a large peripheral envelope deposited at about 40 ka (late Minchin) and (3) a thinner outermost crust formed during a late glacial event. Lake level dropped during interlacustrine stages, sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system, i.e. the present-day salar of Uyuni.