Conference Agenda

The collision of India and Eurasia has resulted in a broad range of structures, from the Himalayan chain to the Tibetan Plateau. The convergence history is characterised by velocities of > 10 cm/yr at collision to current velocities of ~5 cm/yr, of which ~2cm/yr are accommodated at the orogens' front. Our thermomechanical model simulates the collision of India and Eurasia to assess the role of the decrease in velocity, highlighting 4 key phases in the evolution. Phase 1 (50 - 44 Ma) is characterised by a proto-wedge formation, whilst in phase 2 (44 - 32 Ma) the large convergence velocity drives exhumation of crust along a localised channel flow at the front of the wedge, and underthrusting of Indian crust to form a plateau. As convergence slows in phase 3 (32 - 25 Ma), temperature in the orogenic root increases, while compression decreases, allowing the buoyant resurgence of deep crust to form a shallow crustal dome. Further slowing in phase 4 (25 - 0 Ma) allows some hot crust emplacement beneath the plateau. The shallowing of the thermally-activated brittle-ductile transition causes fold and thrust belt (FAT) development at the front of the wedge, while deep burial and exhumation cease. When compared to nature, the deformational phases and their duration are remarkably matched. A switch from large convergence phases, dominated by deep burial, localised exhumation and crustal under thrusting, to slow convergence phases, characterised by large-scale doming of buoyant crust and the formation of a FAT belt, is observed.

The Pamir orogen, part of the Himalayan-Tibetan mountain belt, witnessed a prolonged history of terrane accretion. Two Paleozoic to Mesozoic units have been described from the North Pamir: (1) the volcano-sedimentary North Pamir-Kunlun domain and (2) the metamorphic Karakul-Mazar domain south of (1), representing a Permo-Triassic accretionary prism that formed during the northward subduction of the Palaeo-Tethys Ocean and was later in part subducted beneath the Central Pamir. Both units are part of the North Pamir Terrane.

The Karakul-Mazar unit is interpreted to be emplaced on top of the North Pamir-Kunlun domain as a far distal effect of retro-arc shortening associated with the South Pamir arc in the Middle Cretaceous. Thrusting occurred along the Shala Tala thrust fault. Part of the Shala Tala thrust sheet crops out at the mountain front 90 km ESE of Kashgar in northwestern China. Here, Karakul-Mazar rocks (greenschists, marbles, amphibolites) lie on top of barely metamorphosed marbles and greywackes, which in turn are juxtaposed against Pre-Jurassic volcano-sedimentary units.

South of the village of Bostantielieke we mapped a several 10's of meters thick thrust zone that we interpret to represent a portion of the Shala Tala fault. We present structural field data together with results from K-Ar fine fraction and calcite U-Pb LA-ICPMS dating from the shear zone. Our Upper Triassic to Lower Jurassic ages for the N- to NW-ward transport of the Shala Tala Nappe are much older than previously thought, based on data from the eastern flank of the Muji basin further west.

High-resolution maps of surface strain rates provide crucial constraints on a region’s present-day tectonics, geodynamics and seismic hazard. These maps are now within reach for the Alpine-Himalayan Belt thanks to the COMET-LiCSAR InSAR processing system, which performs large-scale automated processing and timeseries analysis of Sentinel-1 data. We are pairing LiCSAR products with GNSS data to generate high-resolution strain rate maps across Anatolia. Here we investigate what these maps imply for tectonics and seismic hazard. First, we use a probability-based method to pair strain rates and seismic catalogs to estimate the recurrence times of large, moderate and small earthquakes. Iterating over various magnitude-frequency distributions and their governing parameters, and formally incorporating uncertainties in strain rates and the magnitudes of recorded earthquakes, we build a probabilistic long-term-average earthquake model for Anatolia as a whole, including the probability distribution of the largest earthquake magnitude expected. Second, we use arguments from dislocation modeling to identify two key signatures of a locked fault in a strain rate field, allowing us to convert the strain maps to “effective fault maps.” This provides constraints on individual fault systems’ contributions to crustal deformation and seismic hazard in Anatolia. Additionally, we explore how characteristics of earthquake magnitude-frequency distributions may scale with the rate of strain (or moment) buildup, and what these scaling relations imply for the distribution of hazard in Anatolia, using the seismic catalog to evaluate these hypotheses. We also address how to expand these approaches to the Alpine-Himalaya Belt as a whole.

Ultramafic-mafic cumulate rocks often situated at deep root zones of the magmatic systems when they intruded into continental crustal areas. In rare occurrences, they can also intrude at the middle to upper portion of the crust and record the MASH processes developed within the crust. Yıldızdağ Gabbroic Intrusion (YGI) represents one of the rare occurrences of the middle Eocene post-collisional magmatism in Turkey that record the storage of a shallow seated cumulate mush.

YGI intruded within the early Cenozoic flysch sequence of the İzmir-Ankara-Erzincan suture zone (IAESZ) that situated at the northern part of Sivas Basin. YGI displays typical igneous stratification that starts with px-rich lower olivine gabbro at the bottom and it is succeeding by px-poor upper olivine gabbro. Both of these units contain troctolitic enclaves. At the eastern portion of the intrusion, hornblende gabbro units exposed as a marginal zone. All these lithologies cut by relatively younger late stage dykes.

Geochemical data of the YGI display ultramafic to mafic character with typical cumulate affinities. Scattered REE patterns and apparent Eu anomalies also support cumulate character. Pressure calculations based on the mineral chemistry; imply the YGI intrude into middle to upper portion of the continental crust. Partial melting calculations conducted from marginal units and dykes show that bulk of the YGI main body generated by melting of a hydrous peridotitic mantle. Preliminary U-Pb zircon ages display early to middle Eocene crystallization ages for the YGI which is concomitant with basic Early Cenozoic magmatism along the eastern portion of IAESZ.

This contribution concerns Neotethys palaeogeography in the Central Internal Hellenides. Neotethys oceanic crust is represented in the Vardar zone of the Hellenides by the ophiolites of the Almopias sub-zone. Contrary to numerous tectonic models in the literature, we show that the Almopias ocean closed in two episodes.

First, during Late Jurassic and Early Cretaceous, the western Almopias plate partly subducted eastwards while being obducted westwards over the Pelagonian plate. This is especially evident in northern Evvoia.

Second, during Early Palaeocene, the eastern part of the Almopias plate subducted beneath the Paikon-Peonian island-arc complex. This is especially evident in the Vardar zone of Greek Macedonia and partly in the Northern Sporades.

The Pelagonian micro-continent was overthrust by an obducted Almopias ophiolite nappe-sheet during the Late Jurassic and Early Cretaceous, with the emplacement terminating in Valanginian time. As the subducting oceanic leading edge of the Pelagonian plate broke off, it triggered a period of uplift and erosion of eastern Pelagonia as a result of buoyancy-rebound, during and soon after Valanginian time. This explains the vast removal of the obducted ophiolite from eastern-most Pelagonia.

During Early Palaeocene, eastern-most Pelagonia was again overthrust, this time by the Paikon forearc and Paeonian back-arc complex as the Pelagonian plate subducted and crashed with Serbo-Macedonian Europe. The overthrust complex consisted of, possibly Late Jurassic-, but mostly Upper Cretaceous- and Lower Palaeocene- carbonate platform rocks and an imbricated shear-zone-mélange substrate composed of radiolarian cherts, ocean-ridge and island-arc basalts, and intrusive magmatic- and metamorphic-rocks.

Seismic-tomographic-models of the Aegean region (Bijwaard et al. 1998; Hafkenscheid 2004), substantiate two subducting plate-slabs, interpreted here as Almopias lithosphere. The western Almopias slab broke off and began to sink during the Early Cretaceous, and the eastern Almopias slab broke off and sank after the Paikon-Peonoan nappe was emplaced in the Early Palaeocene.

References

Bijwaard H, Spakman W, Engdahl ER (1998)

Hafkenscheid E (2004)

Scherreiks R (2000)

Scherreiks R, Meléndez G, BouDagher-Fadel M, Fermeli G, Bosence D (2014)

Scherreiks R and BouDagher-Fadel M (2020) Tectono-stratigraphic correlations between Northern Evvoia, Skopelos and Alonnisos, and the postulated collision of the Pelagonian carbonate platform with the Paikon forearc basin (Pelagonian-Vardar zones, Internal Hellenides, Greece). UCL Open

Foreland basins can provide records of the geodynamic history of orogens. The Molasse Basin has undergone a still enigmatic phase of exhumation in the Neogene and Quaternary. The timing, magnitude and spatial distribution of exhumation are poorly constrained which hampers our ability to pinpoint the driving force for exhumation of the basin. Here we present a new study that compiles available stratigraphic, low-temperature thermochronology and organic maturity data from the basin and adds new data from the central and eastern part of the basin for which relatively few data were available previously. The data are interpreted using PyBasin, a basin model code that was designed to take into account the uncertainty of low-temperature thermochronometers in sediments that stems from their variable provenance history and chemical composition. The results show up to 900 m of uplift and 2 km of exhumation in the western part of the basin and a decrease in exhumation towards the central and eastern part of the basin. In contrast to previous studies new uncertainty analysis indicates that timing of exhumation is poorly constrained and does not necessarily coincide with climate changes at 5 Ma. The distribution of exhumation suggest that exhumation is not only caused by shortening of the Jura thrust belt but must reflect a long-wavelength process that decreased the load of the European plate in the western Alps.

The Koralpe-Wölz nappe system, as part of the Austroalpine nappe complex, belongs to the crystalline basement of the Eastern Alps and is composed of different metasedimentary complexes with mono-, bi-, or polymetamorphic history (SCHMID et al. 2004; HABLER & THÖNI 2001; THÖNI & MILLER 2009). This nappe system experienced high grade Eo-Alpine metamorphism during or after Alpine nappe stacking. The different complexes were also affected by Permian high-temperature – low-pressure metamorphism. While the metamorphic conditions for several metamorphic events are very well constrained, very little is known about the protoliths of the Koralpe-Wölz nappe system.

Based on U/Pb data of detrital zircons from different metasedimentary complexes, we can give an overview about maximum ages of sediment deposition. Zircon age spectra of sampled metasediments of three different complexes indicate post- Variscan sedimentation with maximum ages of ~ 300 Ma (Koralpe), ~ 330 Ma (Rappold) and ~ 360 Ma (Millstatt). Zircon ages of the Radenthein complex indicate post- Cadomian sedimentation. The main peaks for zircons of the Koralpe complex show Ordovician and Carboniferous ages. One sample from Saualpe (Koralpe Complex) shows a main zircon age peak at ~ 275 Ma which agrees well with the metamorphic “Permian event” (THÖNI et al. 2008). The age spectra for the Rappold, Millstatt and Radenthein complexes are dominated by Cadomian zircons. One micaschist sample from Saualpe and one micaschist sample from the Pohorje massif (Koralpe) are dominated by zircons with an age of around 90 Ma, recording the Eo-Alpine high-grade metamorphic event.

HABLER, G. & THÖNI, M. (2001): Preservation of Permo–Triassic low-pressure assemblages in the Cretaceous high-pressure metamorphic Saualpe crystalline basement (Eastern Alps, Austria). - Journal of Metamorphic Geology, 19: 679–697.

SCHMID, S.M., FÜGENSCHUH, B., KISSLING, E. & SCHUSTER, R. (2004): Tectonic map and overall architecture of the Alpine orogen. - Eclogae geol. Helv., 97: 93–117.

THÖNI, M., MILLER, Ch., ZANETTI, A., HABLER, G., GOESSLER, W. (2008): Sm–Nd isotope systematics of high-REE accessory minerals and major phases: ID-TIMS, LA-ICP-MS and EPMA data constrain multiple Permian–Triassic pegmatite emplacement in the Koralpe, Eastern Alps. Chem. Geol. 254, 216-237.

THÖNI, M. & MILLER, C. (2009): The “Permian event” in the Eastern European Alps: Sm–Nd and P–T data recorded by multi-stage garnet from the Plankogel unit. - Chemical Geology, 260: 20–36.

Ultra-high-pressure (UHP) peridotites found along collisional zones record rare information from deep within the Earth. However, the estimation of depth of origin for these UHP rocks has been controversial. A major controversy remains related to the conjectural proposition of mantle transition zone (410–660 km) origin of the Alpe Arami (AA) garnet peridotite massif in the Swiss Alps. In this contribution, we show micro-textural evidence of precursor majoritic garnet by documenting exsolved rutile, high-Al orthopyroxene, jadeite-rich clinopyroxene and olivine within the AA garnets in this peridotite. We also document an unforeseen texture of olivine with ‘necklace’ like enstatite corona in the kelyphite formed after decomposition of relict garnet. These olivines bear FeTiO₃ and Cr-spinel exsolution needles indicating retrogression from high-pressure Mg₂SiO₄. Thus, the occurrence of retrogressed high-pressure Mg₂SiO₄ with enstatite corona in kelyphite suggests majorite breakdown to precipitate high-pressure Mg₂SiO₄ near mantle transition zone (MTZ) depth. The SiO₂ released during decompression of majoritic garnets reacts with the high-pressure Mg₂SiO₄ to produce the enstatite corona. Our documented micro-textures show high-pressure Mg₂SiO₄ are breakdown product of precursor majoritic garnet, indicating that these micro-textures of the AA peridotite massif are sourced from the mantle transition zone (MTZ).