Roland Oberhänsli

The tectono-metamorphic history of the Valaisan domain from the Western to the Central Alps: New constraints on the evolution of the Alps

Petrographic and structural studies on metasedimentary rocks of the Valaisan domain from the Engadine window (Switzerland) to the Petit St. Bernard (France) show that this entire domain was subjected to high pressure–low temperature conditions during the Alpine orogeny. The pressure and temperature conditions increased from east to west. The metasedimentary rocks of the Engadine window and the Grisons area are characterized by blueschist-facies conditions (12–13 kbar, 350–400 °C, defined by Mg-carpholite and chloritoid occurrences). Toward the west, in the Petit St. Bernard and Versoyen units, the metamorphic conditions reached the eclogite facies (15– 16 kbar, 500 °C). The shortening direction associated with the high-pressure metamorphic event was oriented northwest- southeast all over the Valaisan domain. This direction is compatible with the direction of convergence between the European and Apulian plates in early Tertiary time (65–35 Ma). In the Eastern and Central Alps, blueschist-facies rocks occur below the Austroalpine nappes over an area of 300 × 20 km 2 (from the Tauern window to the Grisons area) and have a thickness around 10 km. This volume of blueschist-facies rocks is in contrast with eclogite-facies rocks of Western Alps that form only a small slice with a thickness of 2 to 5 km. We interpret the difference in volume and metamorphic conditions from east to west as being due to a change of the subduction style. In the east, we assume the formation of a large wedge with a thickness of 40–50 km, such that rocks underwent blueschist metamorphism and were exhumed before collision between the European and Apulian plates. In the west, the Petit St. Bernard and the Versoyen units were metamorphosed and extruded in a subduction channel above the subducting slab. The data further allow us to conclude that the Valaisan domain was an ocean that was independent of the Piemontese Ocean and that had an important place in the youngest phase of the evolution of the Alps before the collision.

Correlation of syn-orogenic tectonic and metamorphic events in the Cyclades, the Lycian nappes and the Menderes massif : Geodynamic implications

The recent discovery of HP-LT parageneses in the basal unit of the Lycian nappes and in the Mesozoic cover of the Menderes massif leads us to reconsider and discuss the correlation of this region with the nearby collapsed Helle-nides in the Aegean domain. Although similarities have long been pointed out by various authors, a clear correlation has not yet been proposed and most authors insist more on differences than similarities. The Menderes massif is the eastern extension of the Aegean region but it has been less severely affected by the Aegean extension during the Oligo-Miocene. It would thus be useful to use the structure of the Menderes massif as an image of the Aegean region before a significant extension has considerably reduced its crustal thickness. But the lack of correlation between the two regions has so far hampered such comparisons. We describe the main tectonic units and metamorphic events in the two regions and propose a correlation. We then show possible sections of the two regions before the Aegean extension and discuss the involvement of continental basement in the Hellenic accretionary complex. In our interpretation the Hellenic-Tauric accretionary complex was composed of stacked basement and cover units which underwent variable P-T histories. Those which were not exhumed early enough later followed a high-T evolution which led to partial melting in the Cyclades during post-orogenic extension. Although the Menderes massif contains a larger volume of basement units it does not show significant evidence for the Oligo-Miocene migmatites observed in the center of the Cyclades suggesting that crustal partial melting is strictly related to post-orogenic extension in this case.

DETACHMENTS IN HIGH-PRESSURE MOUNTAIN BELTS, TETHYAN EXAMPLES

Abstract Using the case examples of the phyllite–quartzite nappe of Crete, the Bundnerschiefer nappe of the Engadine window in the Alps, and the infraophiolitic units of the Saih Hatat window in the Oman Mountains, we describe the tectono-metamorphic signature of large-scale extensional detachments in high-pressure belts. In these three cases retrograde P–T paths were accompanied by the formation of large-scale extensional shear zones that contributed to the exhumation and extreme thinning of the metamorphic pile. The direction of transport along later detachments was parallel to the direction of convergence, detachments being first localized within or near the brittle–ductile transition zone. Below the major shear zone, or between the shear zones in Oman, the deformation regime was more coaxial and involved a significant component of vertical shortening. Metamorphic data, mostly derived from the petrology of metapelites, show that domains characterized by coaxial strain have nearly isothermal decompression paths (type II), while those immediately below the detachments were subjected to more efficient cooling during exhumation (type I). The detachments thus affect both the deformation history and the P–T evolution of the lower plate. The existence of the two types of cooling paths is a good test of the extensional nature of a tectonic contact.

Metamorphism of metasediments at the scale of an orogen: a key to the Tertiary geodynamic evolution of the Alps*

Abstract Major discoveries in metamorphic petrology, as well as other geological disciplines, have been made in the Alps. The regional distribution of Late Cretaceous–Tertiary metamorphic conditions, documented in post-Hercynian metasediments across the entire Alpine belt from Corsica–Tuscany in the west to Vienna in the east, is presented in this paper. In view of the uneven distribution of information, we concentrate on type and grade of metamorphism; and we elected to distinguish between metamorphic paths where either pressure and temperature peaked simultaneously, or where the maximum temperature was reached at lower pressures, after a significant temperature increase on the decompression path. The results show which types of process caused the main metamorphic imprint: a subduction process in the western Alps, a collision process in the central Alps, and complex metamorphic structures in the eastern Alps, owing to a complex geodynamic and metamorphic history involving the succession of the two types of process. The western Alps clearly show a relatively simple picture, with an internal (high-pressure dominated) part thrust over an external greenschist to low-grade domain, although both metamorphic domains are structurally very complex. Such a metamorphic pattern is generally produced by subduction followed by exhumation along a cool decompression path. In contrast, the central Alps document conditions typical of subduction (and partial accretion), followed by an intensely evolved collision process, often resulting in a heating event during the decompression path of the early-subducted units. Subduction-related relics and (collisional/decompressional) heating phenomena in different tectonic edifices characterize the Tertiary evolution of the Eastern Alps. The Tuscan and Corsica terrains show two different kinds of evolution, with Corsica resembling the western Alps, whereas the metamorphic history in the Tuscan domain is complex owing to the late evolution of the Apennines. This study confirms that careful analysis of the metamorphic evolution of metasediments at the scale of an entire orogen may change the geodynamic interpretation of mountain belts.

Anatomy of the Dead Sea Transform from lithospheric to microscopic scale

Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of left-lateral transform motion between the African and Arabian plates since early Miocene (similar to 20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the mu m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull-aparts along them. The damage zones of the individual faults are only 5-20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20-40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere.

Evidence for middle Miocene uplift of the East African Plateau

Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the ∼300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2°, suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa.

Micro-XANES determination of ferric iron and its application in thermobarometry

Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe3+/Fetot (0.2–0.6), whereas only very low contents are present in garnet (Fe3+/Fetot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe3+/Fetot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P–T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe3+ and the actual effect on Mg–Fe2+ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods.

Eclogites within the Menderes Massif/western Turkey

Metagabbros in the core series of the Menderes Massif, for some time considered as post orogenic Miocene intrusives, revealed a strong poly-metamorphic history. The metagabbro bodies exhibit a pronounced zonation. Within their cores, igneous minerals are still preserved. Coronitic textures are interpreted as results of a high temperature, possibly granulitic overprint. The outer parts of the metagabbro bodies mostly consist of strongly to completely retrograded garnet-amphibolites, but occasionally contain relics of eclogites. Petrologic investigations confirmed a high pressure overprint and allowed preliminary P, T estimates (650°C, ≥ 1.3 GPa). The intensity of the HP metamorphism seems to rise from south to north within the Menderes basement, thus pointing to a northward subduction as the cause. The age of the high pressure event still awaits clarification. However, its very existence reanimates the question, how close the relationship between Menderes and the Cyclades evolution might have been.

Mineral supply for sustainable development requires resource governance

Successful delivery of the United Nations sustainable development goals and implementation of the Paris Agreement requires technologies that utilize a wide range of minerals in vast quantities. Metal recycling and technological change will contribute to sustaining supply, but mining must continue and grow for the foreseeable future to ensure that such minerals remain available to industry. New links are needed between existing institutional frameworks to oversee responsible sourcing of minerals, trajectories for mineral exploration, environmental practices, and consumer awareness of the effects of consumption. Here we present, through analysis of a comprehensive set of data and demand forecasts, an interdisciplinary perspective on how best to ensure ecologically viable continuity of global mineral supply over the coming decades.

Preservation of primary volcanic textures in the ultrahigh-pressure terrain of Dabie Shan

Delicate primary volcanic features such as ash layers, volcaniclastic breccias, and pillow lavas with amygdaloidal rims have been identified in a terrigenous volcanosedimentary sequence forming part of the ultrahigh-pressure unit of the Dabie Shan, China. The presence of coesite relics in crosscutting dikes confirms an ultrahigh-pressure evolution for the entire sequence. Evaluation of the probable reaction history for zeolite compositions as supposed for the amygdaloidal rims reveals a scarcity of reactions at very high pressures. Such special conditions of low reactivity and fluid-conserving reactions, combined with a lack of directed stress, have allowed the remarkable preservation of these primary volcanic features despite deep subduction.