Aegean Sea Geology
A Synthesis of the Geology Seen During Geol-373
Regional Overview
One of the defining features of the Aegean Region is the subduction of the African Plate underneath the Eurasian Plate, beginning approximately 100 Ma. This subduction is initiated by rifting from a triple junction between the African, Arabian, and Somalian plates that is found below the Red Sea. In addition to assisting the African Plate's movement, the triple junction also pushes the Arabian Plate northward. The northward motion of the Arabian Plate has resulted in multiple strike-slip faults, the most notable of which are the North and East Anatolian Faults. These strike-slip faults have caused the Eurasian Plate to further accelerate downwards. This combination of strike-slip faulting and subduction has caused the Eurasian Plate in the Aegean region to greatly extend, with its crustal thickness being only 15-20 km and over 250 km of extension since the eocene. Earthquakes are also prevalent in the region, with many occurring along the major plate boundaries surrounding the Aegean.
The modern subduction zone is situated just off the Northern Coast of Africa. However, the subduction zone has not always been located here. It was previously located further north in the Aegean. However, slab rollback has caused the zone to gradually progress south, assisting with the crustal extension in the area. Additionally, slab tear around 10-12 Ma further accelerated the extension of the region. Currently, Crete is moving southeast at a rate of 35 mm/yr, with 25 mm being attributed to divergence from Eurassia and 10 mm being attributed to convergence with African Plate.
Forearc Region
Tectonostratigraphy
On the island of Crete, and throughout most of the forearc region, exists 5 prominent formations. The lowest unit is the Platenkalk, a platy metamorphosed unit deposited from the Permian to the Oligocene. The oldest parts or shallow-water dolostone before transitioning to deep-water limestone and chert before being topped by meta flysch. The next unit is the Phyllite-Quartzite Unit. The P-Q is composed of primarily Phyllite (metamorphosed mudstone) and Quartzite (metamorphosed sandstone) but becomes more carbonate-heavy as you move upsection. The top of the unit is topped with Gypsum, and the entire unit's deposition is dated from the Carboniferous to the Triassic. Above this lies the Tripolitza Unit, a massively bedded light grey limestone topped with flysch and depositionally dated from the Triassic to the Oligocene. Above the Tripolitza is the Pindos, a rhythmically bedded heterogeneous unit containing chert, limestone, and some sandstone, is topped with flysch, and was deposited from the Triassic to the Eocene. The final unit is the igneous and metamorphic Uppermost. The only one of the units to have formed on the Eurasian plate, it was metamorphosed as the other units were deposited. All of the units are separated by detachment faults with a thrust sense of motion, excpet for the P-Q and the Tripolitza. These two formations are seperated by the Cretan Deatachment Fault, a reactivated detachment fault with a normal sense of motion.
History of Cretan Formations
All of the sedimentary formations on Crete were initially deposited at sea. The Pindos, Tripolitza, and Plattenkalk formations were all deposited beside each other, while the P-Q was deposited and then covered by the Tripolitza. and Plattenkalk. As the subduction zone moved further south over time, as a result of slab rollback, these formations were subducted one by one. The Pindos was the first formation to be subducted, followed by the Tripolitza and the P-Q. The Plattenkalk was the final formation to be subducted. All of these formations would slide past the previously subducted formation, forming a thrust fault between formations. Once subducted, these plates would be underplated and metamorphosed. This underplating resulted in the initial uplift and emergence of Crete. Crete then became so thick that the mass of the island and gravity caused the island to "crack," which resulted in normal faults across the island. Normal faults cause an extensional environment, which in turn exhumed the formations to the surface.
Modern Volcanic Arc
History of Volcanism in Santorini
Subduction of the African plate initially began during the Jurassic-Cretaceous periods. Through subduction, water was brought down from the surface, lowering the melting temperature of mafic rocks and causing volcanism. Around 9-12 Ma, the subduction rate increased, correlating to an increase in volcanism. The oldest record of volcanic activity near present-day Santorini can be found at the Christiana Islands. These islands, located southwest of Santorini, have volcanic activity dated to 2.5 Ma. On Santorini itself, the earliest evidence of volcanism can be dated from 2 Ma-600 ka. Volcanic activity in Santorini can be broken up into 2 categories. The first category is the 12 Plinian Eruptions. These eruptions can be characterized as major volcanic eruptions, with the erupted material being felsic. The first Plinian Eruption occurred at 360 ka, and these eruptions have a recurrence interval of 30 ka. The most recent Plinian Eruption was the Minoan Eruption, occurring in roughly 1615 B.C.E. In between Plinian Eruptions were other volcanic activity, such as sub-plinian eruptions and basalt flows, that were more mafic.
Description of Volcanic Deposits
There are a variety of volcanic deposits on Santorini. One of the most common ones is Pumice. Pumice is formed when ash is shot into the sky during a major volcanic event. This ash falls to the ground and solidifies, creating a layer of Pumice. Because all of the Plinian Eruptions on Santorini were felsic, the ash and pumice were felsic as well, resulting in a lighter color. Another prominent volcanic deposit on Santorini is from the pyroclastic flow. A pyroclastic flow occurs after the ash that becomes pumice is evacuated from the volcano. This evacuation changes the pressure in the volcano, causing a lava flow over the ground that is driven by gravity. As it flows, the felsic lava rips up chunks of angular, mafic material known as volcanic blocks and carries them with the flow. Eventually, the flow slows and solidifies, which can be seen in the rock record today.
Change in Volcanism with Time
The Santorini Plinian Eruptions can be broken up into two distinct cycles, with the first cycle containing five eruptions and the second cycle containing seven eruptions. The initial eruptions in each cycle had a lower SiO2 content than eruptions later in the cycle, which is indicative of a more mafic eruption. Throughout the course of an eruptive cycle, the melts became more felsic with the SiO2 content increasing. This can be attributed to fractional crystallization. As a melt rises through the crust between plinian eruptions, its temperature would gradually cool. This cooling temperature would cause more mafic minerals to solidify and fall to the bottom of the melt, as mafic minerals have a higher cooling temperature than felsic minerals. This would cause the melt to become more felsic, causing the next eruption to be more felsic.
Backarc Region
Nature of Metamorphic History
The most prominent geologic formation in the backarc region is the Cycladic Blueschist Unit. The CBU as a whole was buried to eclogite facies conditions of 550 Celsius, 70-80 km of depth, and 20 kbar of pressure. At the surface however, the CBU is composed of a variety of facies and types of metamorphic rocks, such as eclogite, blueschist, greenschist, and marble. The marble is a result of a high content of metamorphosed CaCO3 in the rocks. The other rock types are a result of retrograde metamorphism, metamorphism that is a result of decreasing pressure and temperature and the rocks adjusting to that decrease. Two main factors control the rate of retrograde metamorphism: time and fluid content. Because of these factors, different parts of the CBU took different paths back to the surface, experiencing different degrees of retrograde metamorphism.
Burial and Exhumation of Rocks
The protoliths of most of the metamorphic rocks in the cyclades were initially deposited at sea. These sandstones, carbonates, and mudstones were deposited mainly side-by-side on the African plate and one-by-one were subducted under the Eurasian plate. They were brought to depth where they experienced their peak high pressure and low temperature metamorphic conditions. Then they were underplated as another set of deposits were subducted and underwent metamorphism. After underplating, the newly metamorphosed rocks were exhumed. This exhumation was driven by more underplating of different rocks and formations. As the rocks were exhumed, they underwent various levels of retrograde metamorphism before reaching the surface, where they can be seen today.