The evolving anatomy of a collapsing orogen

Abstract

The Tethys Oceans separated Africa and Arabia from Eurasia, and India from Asia. Closure of the Tethys started in the Jurassic and led to the Alpine-Himalayan mountain chain. This thesis will focus on the Aegean segment of this mountain belt.

The Aegean region is occupied by a stack of nappes that were emplaced during the closure of the Tethys. In the late Eocene, the oldest and most internal nappes of the nappe stack started to fall apart, or collapse: late-orogenic extension fragmented the nappe stack, leading to the formation of metamorphic core complexes and associated sedimentary basins, while stacking of new nappes continued at the active margin.

This thesis focuses on the question whether it is possible to identify the large-scale geodynamic processes that led to the collapse – or fragmentation – of the Aegean segment of the Alpine orogen. To best identify these, the timing, magnitude and direction of vertical and horizontal motions in the entire Aegean region were reconstructed by paleontologic and paleomagnetic measurements and structural and sedimentologic analysis, since the possible large-scale processes are likely to lead to a specific spatial contemporaneous and evolving distribution of these motions. Thus, we reconstructed the evolving anatomy of the Aegean region since the onset of nappe stacking in the Jurassic.

We investigated whether the fragmentation can be subdivided into different phases, and if so, we constrained the timing of onset and duration of these phases. Therefore, we reconstructed the palinspastic evolution of the Aegean region since the Eocene and compared it with the structural, metamorphic and magmatic history of the region. This led to the identification of a number of phases: 35-23 Ma, 23-15 Ma, 15-8 Ma, 8-3.5 Ma and 3.5-0 Ma. The first two phases are characterised by – probably roll-back induced – N-S extension, the formation of metamorphic core complexes and the onset of volcanism.

Around 23 Ma, two nappes decoupled from the subducting slab. Afterwards, they were extended and exhumed between 23 and 15 Ma in two metamorphic core complexes, the tectonostratigraphically higher one of which experienced a high-temperature overprint as a result of exhumation of hot material along the lower one. Around 15 Ma, the onset of lateral extrusion of Anatolia led to clockwise rotation and reestablishment of compression in western Greece. From 8 Ma onward, the overriding Aegean lithosphere started to spread out over the underthrusting lithosphere, leading in the northwest to collision with Apulia. This collision was established around 3.5 Ma, after which the ongoing spreading of the Aegean region led to interfering curved extensional basins such as the Gulf of Corinth-Saronic Gulf composite basin system. The timing of these phases correlates well with Mediterranean wide, large-scale palinspastic and tectono-stratigraphic phases