Field trip to the French and Italian Alps
June 15 to 21, 2014 / day 1: Massif de l’Esterel
Monday June 16
Stage one of the Wilson cycle Continental rifting – tectonic sedimentation and volcanism in the Permian Massif de l’Esterel
Today we observe continental rifting from the Permian. It is lying with an angular unconformity on the Variscan basement with lineations in a N-S direction, the rift itself being E-W. The basement is composed of metamorphic and plutonic rocks. In this region we have fluvial sediments, ryholitic and basaltic volcanic sediments with rift structures and rifting processes.
Stop 1: Base of the Permian succession, early continental conditions with fine fluvial deposits.
Here we found a stratified outcrop with a channel, mostly fine sandstone interspersed with clay and a fining upwards sequence, also wood has been found in some parts. This shows a braided river environment in a large fluvial channel. We infer from this that there was a sluggish current in this location indicating low relief on a gently sloping plain. The mature remnants of the eroded Variscan mountains have been deposited in streams within a peneplained region, this was a time of extended tectonic stability, where the tectonic plates are above sea level in equilibrium.
Stop 2: Rhyolitic ignimbrites – Pyroclastic flows (nuée ardentes)
At this road cutting we saw two similar well defined units separated by a finer darker layer. The two similar units were unsorted with no internal structure, no flow banding and appeared frozen before compaction. There are some voids where pumice has weathered out. Françoise explained that under thin section we would see broken glass shards, fragments of crystals and pumice with vesicles from 50mm to around 1mm within a finer matrix.
This shows we have an ignimbrite, a deposit formed by the high temperature gas and pumice rich ash mix of a pyroclastic flow event. Ignimbrites are poorly sorted with lapilli dispersed in an ashy matrix.
Pyroclastic density currents are referred to as "flows" or "surges" depending on particle concentration and the level of turbulence and are also known as nuée ardentes (fiery clouds). The finer darker volcaniclastic layer had no cross stratification which would have classified it as a surge deposit and did not have enough visible stratification to classify it as an ash fall deposit. (Pyroclastic fall deposits consist of well sorted lapilli with little ash)
A compositional analysis reveals abundant quartz and alkali feldspar, showing we have a rhyolitic ignimbrite. Rhyolites are more common and voluminous on the continents than in the ocean basins and pyroclastic flow deposits spread out close to the eruptive vents. These observations lead us to confirm active continental rifting was taking place in the region.
Stop 3: Basaltic lava flows
At this road cutting, there are several lava flows which show an assortment of rounded and angular clasts, some with white/grey hyrothermal crystals formed via exolving gas percolating in the flow. The texture is fine grained crystalline with some phenocrysts of feldspar, pyroxene and augite – a dolerite. Plagioclase occurs as subhedral tabular laths up to 2.0mm. There is visible haematite staining possibly due to weathering of magnetite to a reddish brown.
We see no olivine and no mantle xenoliths. There are many lava flows one after another with no flow breccia from the breaking up of the cooling surfaces (aa), all of the flows are dolerite and above them are fluvial sediments. We can speculate that the source is from dykes along the fracture to the north.
Françoise indicates that it is perhaps surprising to find a doleritic texture in a permian lava flow and that trace elements show that we have an intermediate calc-alkaline / pholeitic basalt that is typical of an alkaline basalt source.
We discuss two theories for the alkaline basalt formation: one, a hot spot from lower mantle and two, an alkaline basalt from the continental mantle lithospehere. Alkaline (intermediate calc-alkaline) basalts can come from melting of continental lithosphere via decompression melting. This gives an intermediate signature, so perhaps comes from the base of lithosphere close to asthenosphere (asthenosphere being the residue of the primitive mantle)? We conclude from the evidence that continental crust is being recycled in the mantle and subduction of continental crust has occurred.
Stop 4: Panoramic view, rift zone, rhyolitic cliffs
From here we can observe the rhyolite valley cliffs with N-S extension. Typical in rifted systems is that instead of having planar fault planes, the fault is curved. The fault surfaces we observe here are dipping into the valley. Françoise informs us that we have later sediments deposited in the basin of the valley, while the faulted walls of the valley are alternate deposits of sediment and rhyolitic deposits sitting uncomfortably on basement granite. This is a typical structure found in locations such as the North Sea Basin or the Rhine Valley.
Stop 5: Late continental coarse detritic deposits.
Here we see a series of conglomerate sandstone pebbles (from the Permian) and volcanic clasts. Françoise explained that the Permian sandstones layered with rhyolites were in relief and exposed at the surface and eroded. The shoulder of the rift was uplifted resulting in layered sediments being deposited as the rocks at surface were being weathered.
We deduce that the relief was forming during active phases of sedimentation and volcanism – the uplift was contemporaneous with the sedimentation and volcanism. At the large scale a slight uplift of the lithosphere occurred - around 1500m, during extension. Decompression melting forced the uplift but the exact model of its behaviour is not clear.
Stop 6: late continental deposits – Arenaceous deposits
Some stratification visible here in this an arkose textured unit from fluvial flows in channels. These are locally formed volcanic sediments formed by the alteration of granite in a tropical climate creating a volcanic arenite. The tropically weathered granite has been transported in a mass flow locally and likely has a quartz cement. We are seeing local mass flow transportation via fluvial action from the uplifted shoulder of the rift.
Mark Biswell, London branch
Day 2: Massif d'Esclangon
Tuesday June 17
On Tuesday we continued our exploration of the Wilson cycle in a visit to the Esclangon Massif which shows the effects of both continental extension (stage 2) and continental collision (Stage 5a) on the western margin of the collisional system.
The Massif consists of two main units: the lower one is autochthonous and consists of Mesozoic to Cenozoic folded sediments (the relationship is one of angular unconformity). The upper unit of the Massif overlies the lower unit in a major thrust fault, the Nappe de Digne.
During continental extension, the Mesozoic formations of the lower unit (Trias to Cretaceous) formed on the western margin of a marine continental basin. The Mesozoic formations of the upper unit, the Nappe de Digne (Trias to Lower Jurassic), represent distal marine deposits laid down with those of the lower unit just before the opening of the ocean (stage 3 of the Wilson Cycle).
During continental collision, the Mesozoic formations were strongly folded, and those of the upper unit were detached and included in the nappe that was thrusting south-westwards. The Cenozoic sequence of the lower unit consists of detritus deposited and folded as topographic relief developed in front of the moving nappe, giving the unique and spectacular structure of the Velodrome de l’Esclangon, These sediments (molasse) are continental (Eocene to Oligocene) and later marine (Miocene to Pliocene) and represent the erosion products of the mountain range during the collision stage.
Our objective was to reconstruct through our field observations the tectonic structure of the collision stage and understand the relationships between tectonics and sedimentation on this western margin of the orogenic cycle in its later stages.
Locality 1 (44.217, 6.276): At the Clue du Peroure, we observed reddish vertical Eocene breccia abutting low-angled yellowish beds of alternating laminate and massive material (Figure 1).
The beds here are inverted, as thick Jurassic limestone beds (Tithonique) lie on top of the Cretaceous; the latter is the source of angular fragments in the breccia. Small shear structures in the thinner beds confirm the inversion.
The Eocene breccia is localised detrital molasse in front of an authochthonous fold, formed before the Upper Cretaceous as there are no rocks of this age here. Madame Chalot-Prat retro-engineered the genesis of the locality stage by stage to take us back to the passive margin of a Jurassic-Cretaceous marine basin.
Locality 3 (44.213, 6.282): At this elevation, we could appreciate the structure of the thrust at map scale, with large triangular blocks of Jurassic dragged along beneath it like contact breccia, overlain by stratified purple and ochre Trias (this contains gypsum, a major lubricant for the nappe) and by Lower/Upper Trias that has been transported east, and grey Lias sediment above the Trias, all deeply folded in three dimensions (Figure 4). Below the contact we observed a recumbent anticline (we were looking down the hinge) plunging beneath Eocene-Oligocene. This hog-backed structure is composed of the beds that we examined in Locality 1; it was an instructive example of how important three-dimensionality is in structural geology.
Locality 4 (44.208, 6.234):
At Bayeul summit, the erosion of the nappe has resulted in a tectonic window, the Velodrome, which reveals the heavily folded and inverted multiple beds of a vast fluvial fan system (Figure 5, next page). These deposits were folded in the process of sedimentation and in-filled basins created by buckling, a process intensified in the mid-Miocene, as the Adriatic plate pushed the sea floor sediments, resulting in a fold with vertical limbs in this locality, which was overturned by the nappe to produce a reverse syncline.
As we ate our sandwiches, the thunder, lightning and torrential rain burst over our heads; this had been threatening most of the morning; our final exercise, the drawing of a cross-section integrating sedimentation and tectonics was abandoned, and we beat a rapid retreat.
Locality 5 (44.119, 6.234): At the final locality we inspected the Dalle d’Ammonites, a continuous slab of Lower calcareous Lias at the foot of the Digne nappe (Figures 6 and 7). The steeply dipping bedding planes are covered with over 1500 ammonites including Arietites, and are part of the distal sediments transported by the thrust.
Text: Elisabeth Davenport / Images: Stuart Swales (East of Scotland branch)
Day 3: Saint-Véran
Wednesday June 18
From oceanic plate spreading to oceanic plate subduction
An amazing day despite the snow and cold, but a very welcome lunch and hot chocolate stop in the little refuge by Lac Blanchet. Stunning views from this vantage point!
We started from the car park a few miles on from the village. We passed a disused marble quarry and just a little further on we passed a copper mine. Not far on we saw our first blueschist.
The St Véran area, from Chapelle de Chapelle de Clausis to Lacs Blanchet consists of upper Jurassic (160-150 Ma) series of sedimentary, magmatic and mantle rocks representative of the uppermost part of the oceanic lithosphere. This ophiolite is metamorphic (high pressure-low temperature Blueschist facies) and more or less folded.
Plate tectonic context
The St Véran area belongs to the oceanic suture, the formations of which were subducted and strongly metamorphosed during the 4th stage, then exhumed and retromorphosed during the 5th stage of the orogenic alpine cycle. This oceanic unit is representative of the bottom of the ocean, named Alpine Tethys, during active oceanic spreading during the upper Jusassic.
Along the trail, facing the Chapelle de Clausis to the Lacs Blanchet, we observed different highly metamorphic textures and mineral assemblages within mantle, gabbro, basalt, limestone and flysch, and deciphering of their metamorphic paths and their original geometic relationships within the oceanic lithosphere.
The blueschist as a rock type is defined by the presence of the minerals glaucophane + (lawsonite or epidote) +/- jadeite +/- albite or chlorite +/- garnet +/- muscovite in a rock of roughly basaltic composition.
It often has a lepidoblastic, nematoblastic or schistose rock microstructure define primarily by the chlorite, phengitic white mica, glaucophane and other minerals with an elongate or platy shape.
All in all a very interesting day and we wondered if it could any get better… it did!
Text: Lilian D’Mello / Images: Stuart Swales
both East of Scotland branch
Day 4: Chenaillet Massif
Thursday June 19
Having spent the night in Claviere just over the Italian border on Wednesday night, we headed back into French territory on Thursday morning to start what would be our most physically challenging day of the field trip, with the aim of identifying evidence for oceanic rifting and subsequent continental collision as part of our continued exploration of the Wilson Cycle.
The Chenaillet Massif is a remarkably undeformed obducted upper Jurassic ophiolite which forms a nappe overlying the St Véran area we had visited the previous day. It belongs to the Alpine orogenic suture and represents the collision stage of the Wilson Cycle, with the ophiolite itself representing a slowspreading ridge similar to the Atlantic ridge.
Our first stop of the day was to observe a pile of pillow lava tongues, indicative of subaqueous extrusion of lava. We observed it perpendicular to the flow direction and Françoise described their size, geometry and features, as well as getting us all to draw sketches of what we saw – always a useful skill to develop for a geologist.
Our next stop was to observe more pillow lavas but this time they were visible parallel to flow direction and so we were able to see their shapes from another angle, highlighting their shapes as tubes flowing on a slope. By combining our observations with those of stop 1 we were able to get a good understanding of the shape and movement of pillow lavas in three dimensions.
Spherulites were also observed and this sparked a lively and interesting discussion about their use as a diagnostic feature, with Françoise explaining that their distance from the rim of the pillow lavas was related to the speed of quenching of the lava. Pillow breccias were also observed evidencing pillows broken apart by fractures, and these usually occur on the steeper slope near the vent while whole pillows are preserved on the gentler slope. Finally, the spatial relationships between tongues were used to determine the relative ages of different pillows.
The third stop was a contact between gabbro and basalt, with the sinuous contact between the two cited as evidence for the gabbro still being relatively plastic when the basalt dykes cut across it.
After a steep climb and a welcome rest for lunch, our next stop was for banded gabbro, where both brittle and ductile deformation were visible, and these kinematic indicators allowed a sense of movement in the shear zone to be determined. Both the mylonites and cataclasites present formed part of a detachment fault, explaining both why these ductile and brittle deformation features are present and how the gabbro arrived at the surface.
These detachment faults were dated 155 Ma in the middle to upper Jurassic, and the fact that they had been recently exhumed explains the sinuous contact with the basalt dykes observed at the previous stop. The source of the basalt however was not fractional crystallisation but had a deeper origin and it was tholeiitic.
The fifth stop was to observe some peridotite breccia within a calcareous mudstone, and here we found evidence of cross-bedding due to strong currents at the mid-ocean ridge as well as sedimentary deposits of serpentine sand. The sixth stop also took in some sedimentary features, with talus breccia due weathering of the mantle following exhumation along a detachment fault.
Our next stop took us atop the Chenaillet Massif itself, and after taking a moment to catch our breath, take in the amazing views and pose for a group photo, Françoise showed us the tooth and comb features where series of dykes forming the teeth of the comb were visible parallel to each other, with the oldest down slope and the youngest up slope, and these perpendicular to the branch of the comb.
Françoise explained that with students she took samples from the different dykes along the sequence and that the geochemisty of the parallel dykes matched, providing solid evidence that what we could see was indeed an undeformed section of a mid-ocean ridge.
After coming down from the Chenaillet massif we made our way to the final stop of the day, the falaise du Collet Vert where a vertical cliff face composed of in situ pillow lavas and dykes was observed. This wholesale displacement of a pillow lava sequence can be explained by exhumation of the mantle.
Following a long but rewarding day in the Chenaillet Massif where Françoise had shown us some remarkable geological features, we made our way back to our hostel for some well-earned rest.
Text: Emmanuel Kavanagh / Images by Stuart Swales, unless otherwise indicated
Day 5: Ivrea Massif
Friday June 20
Setting off early in the morning we whizzed across northern Italy from Sestriere past Torino to the small town of Varallo, west of Lake Maggiore. It made a real difference having the side window of the minibus fixed at last! The bulk of Gran Paradiso, one of the major Internal Crystalline Massifs of the western continental margin, dominated the northern horizon as we crossed the Piedmont plain. Also visible from here was the distinctive peak of Monte Viso with composed of UHP rocks such as eclogite. These were part of the Internal Zone of the western margin, which sadly we had to omit on this trip. Leaving our overnight things at the hostel in Varallo we then set off westwards, up the Val Sesia, for the day’s hands-on geology.
The first stop was just to the east of the village of Balmuccia. Here, we had our lunch, and Françoise explained more about the setting of the Ivrea Zone, which is a nappe of the eastern continental margin that contains some subcontinental mantle rocks, above which are large (40 km long x 10 km thick) gabbro bodies of Permian age. As everything has been folded, the gabbros may have been thinner originally but still probably 7 km thick. Above these, and to the east, granulitic lower crust is present.
Finally one would find the upper crustal units with Permian granites and volcanics – also with Permian rift sediments as we saw on Day 1 on the Esterel Massif. It is believed that the deeper (mantle and granulite) rocks of the Ivrea Zone were uplifted to higher crustal levels towards the end of the Variscan Orogeny.
Going down rocks and a ladder to the River Sesia, there is revealed an exceptional riverside outcrop of ochreous mantle, well polished by water when the river runs high. The mantle rocks were veined by grey-green and grey-brown veins generally up to 10 cm wide (largest seen was 20 cm wide), mostly sub-parallel, with some cross-cutting at low angles. Splashing water on the rocks helped to improve the contrast and see the colours better, though with it being such a lovely hot day it quickly evaporated!
The edges of the veins were not as sharp as first thought, showing blending with the host rock. The 3D nature of the outcrop showed that the veins were pervasive and vertically extensive through the outcrop. Small-scale faults could be identified where they had displaced the veins, however few faults seemed to displace more than one set of veins. Some of the veins exhibited small-scale open folding, and a schistosity could be discerned in some of the veins. Some veins thinned and bulged across their length, with one wider vein abruptly terminating in a sharp butt-end.
Françoise explained that the different colours of the veins reflected their differing compositions. The greener veins were largely diopside (clinopyroxene) with some spinel and some enstatite. The greyer veins had an enstatite (orthopyroxene) matrix with many cm-scale black clots of spinel. Rarer veins with sinuous contacts contained black amphoiboles – possible metasomatism?
The mantle host rock here is peridotite. Analyses had shown this to be approximately 70% olivine, 20% diopside and 10% enstatite – i.e. a lherzolite, but locally varying to harzburgite. mm-scale square and rectangular brown phenocrysts could be seen on weathered surfaces. The olivine was weathering to an ochre colour but surprisingly (to me at least) was shown to be clear grey to black in fresh samples. As in their corresponding veins, the diopsides were green and the enstatites clear grey. There is apparently a little spinel to be found in the peridotite but it is microscopic. Closer inspection of the peridotite close to the edges of veins showed scattered phenocrysts that have been derived from the corresponding vein (Di/En).
The second stop of the day was a short distance downstream from the first, and was intriguingly labelled ‘Supervolcano’ at the car park – it is apparently a quite new UNESCO Geopark (see www.supervulcano.it). We wandered along a wooded path to another excellent riverside exposure, this time of a layered gabbro.
This had an isotropic texture in places, being well zoned in others and was composed of brown amphibole, augite and plagioclase feldspar, the phenocrysts being best seen in the non-layered zones with euhedral amphibole ‘lozenges’ , augite ‘sticks’ and xenomorphic plagioclase feldspars.
The most common layering seen on this outcrop showed a sharp-based dark layer grading up into lighter, more plagioclase-rich, layers over about 10 to 20cm.
Quite striking were the tight-to-isoclinal folds, with the layers thinned on the fold limbs. These folds apparently have similar axes to those seen in the mantle rocks of the first stop. One large fold had developed an axis-parallel schistosity in the gabbro.
We discussed the calcic nature of the brown amphibole and augite and thought that it was most likely that the plagioclase would be quite calcic too, giving a gabbro. Looking carefully at the sharp-based dark layers again, we found that sometimes it was augitedominated, sometimes predominantly brown amphibole, which led onto a revisiting of ternary phase diagrams and how the first crystals to form from a liquid can be quite sensitive to the original bulk composition of the gabbro’s parent magma.
Magmas of gabbroic composition have a low viscosity (unlike silicic magmas) and so allow crystal settling as seen so clearly here. The plagioclases in this gabbro were always crystallising last, surrounding and penetrating interstices between the euhedral mafic minerals (hence the designation xenomorphic).
Françoise told us that this particular gabbro body was about 30 km long by 7km thick. She also explained that many of the gabbro bodies in the Ivrea Zone were intruded sideways as thick sills along detachment faults, this being confirmed by geophysical investigations, and not as bottom-fed plutons. This is likely to be related to the Permian rift-initiation processes as seen in the Esterel on the opposing margin of the eventual rift.
We concluded the visit to this stop by wandering upstream examining the profusion of different types of rocks brought down in boulder form by this powerful river. Many gneisses, granitoids and schists were found, but I think we were unlucky not to find any eclogite.
At our group evening meal in Varallo we took the opportunity to profusely thank Françoise for the wonderful field trip and Elisabeth and Beatrice for their excellent organisation and driving. Well done everyone involved! My own thanks go to everyone on this trip for the good company and interesting and stimulating discussions we had during this week.
Stuart Swales (East of Scotland branch)