19/11/2014
19/11/2014
Julian Swinkels
4119355 | j.r.a.swinkels@students.uu.nl
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Mare Nostrum
A report looking into the geodynamics(1), THE PALEOENVIRONMENT(2) and volcanism & earthquakes(3) of the (eastern) mediterranean area
Julian Swinkels
4119355 | j.r.a.swinkels@students.uu.nl
-------------------------------------------------
Mare Nostrum
A report looking into the geodynamics(1), THE PALEOENVIRONMENT(2) and volcanism & earthquakes(3) of the (eastern) mediterranean area

Table of content Introduction 1 Geodynamics 2 Volcanism and Earthquakes 4 Paleoenvironment 6 References: 8

Introduction

The Mediterranean Sea has been one of the most profound areas for geologists to study at. Amongst each other they also refer to the sea as ‘Mare Nostrum’ which is Latin for ‘our sea’. It is an area where many different types of tectonic plates come together, moving in different directions. It also has the perfect latitude to make it a sensitive area regarding astronomically induced oscillations. In the past it has been a fascinating area for geological research and further analysis of the area would contribute greatly of many geological process in an accurate time-frame. This report will first explain the geodynamics of the area focussing on the subduction processes in the eastern Mediterranean and how extension is possible in an overall compressive state. Secondly the volcanic activity and earthquake activity in the Aegean domain will be addressed, looking at how they are related to tectonic plate configuration and finally discuss the Messinian Salinity Crisis which as part of a dramatic change in paleoenvironmental conditions.

Geodynamics

Geodynamics is a subfield of the earth sciences that looks into movements and processes of the earth’s mantle. Some of the predominant topics include plate tectonics, seafloor spreading, mountain building, volcanoes, earthquakes and faulting. The Mediterranean is a particularly interesting case regarding geodynamics as it is land-locked between the collision zone of Europe and Africa and remains to be tectonically active. The tectonic plates involved have faced both a break-up and collision. From this we can derive that the history of the geodynamics in the Mediterranean sea is rather complex. Till today the Mediterranean is one of the most fascinating area’s for geophysicists to work at. It contributes significantly to the understanding of geodynamics. This part of the report will clarify why the Mediterranean is such a fascinating and unique region by explaining the evolution of the Mediterranean area focusing on the East and how subduction processes in the Hellenic arc have caused extension in an overall compressive state.
One of the primary geological features that gave insight into the history and movements of the area are the mountain belts. They hint towards a collision between the African plate and the Eurasian plate. It is estimated that there was a conversion of 400-500 km in the west and 1500 km in the east. The Alps are a classic example of consequences that can arise due to conversion , located around the north-eastern part of the Mediterranean, they provide evidence for subduction of the oceanic plate due to the northwards motion of the African continental plate (Krijgsman et al 2002).
Figure 1: Lateral migration of a slab-detachment. When the slab starts to detach this could lead to roll-back of the subduction plate. In A the tear starts eventually leading to B the where the slab is almost completely detached.
Figure 1: Lateral migration of a slab-detachment. When the slab starts to detach this could lead to roll-back of the subduction plate. In A the tear starts eventually leading to B the where the slab is almost completely detached.
What makes the Mediterranean such a unique area is the Aegean region. This area consist of Greece, the Aegean Sea and western Turkey. While the African plate and the Eurasian plate are converging, the Aegean region has shown a major tectonic phase of extension. After many years of scientific research this enigmatic process can now be explained by slab-roll back of the subduction zone (Wortel). When the plate convergence rate is zero or close to zero the potential energy implicit in this instability cannot be released by sinking of the lithosphere in the subduction zone’s non-vertical direction. Instead the subduction plate starts to sink vertically. The upper-plate, in this case the Eurasian plate reacts to this reduced pressure by extension (Wortel). The Hellenic Trench has thus been undergoing a southward migration creating space for the Aegean region to extend.
Figure 2: The motion of the Aegean plate and that of all the surrounding plates.
Figure 2: The motion of the Aegean plate and that of all the surrounding plates.
Other scientists came up with an alternative explanation for the extension of the Aegean Sea plate. They point out that ever since the continental collision of Eurasian and Arabia, Turkey has kept on moving westward causing all kinds of deformations and extrusive processes. However, all kinds of numerical modelling together with observations have indicated that the primary cause has to do with an out-ward pulling force associated with the Hellenic trench (Wortel). Present-day activity and data from Global Position Stations (GPS) have shown that the westward push of Turkey remains to be a very significant background process that improves the fit of displacement caused by the slab-roll back.
Regarding the present day movement research has shown similar results. The Anatolian plate continues to cause stress by slowly pressing westwards and the Aegean plate is now be considered as an individual micro plate as it moves at a much higher velocity than the Anatolian plate does. The Hellenic Trench also continues to migrate southwards. If there are no unexpected changes in the future it is expected that the oceanic plate that is currently land-locked between the four continental plates will disappear completely. The Aegean Sea on the other hand, could potentially save the marine ecosystem that can currently be found in the Mediterranean Sea.

Volcanism and Earthquakes

A chain of volcanoes is known as a volcanic arc. They develop behind an accretionary prism. The magma that feeds these volcanoes forms above the area where the downgoing plate reaches a depth of roughly 150 km. If the volcanic arc forms where an oceanic plate subducts beneath continental lithosphere, the resulting chain of volcanoes grows on the continent and forms a continental volcanic arc. If, however, the volcanic arc forms where an oceanic plate subducts beneath another oceanic plate, the resulting volcanoes form a chain of islands known as a volcanic island arc. This arc may grow on a piece of continental crust that split away from the main continent when a back-arc basin formed (Stephan Marshak).
Figure 3: The South Aegean Volcanic Arc
Figure 3: The South Aegean Volcanic Arc
The Aegean region is located between the African and European lithospheric plates that are currently converging. It is , where the tectonic activity is relatively high and there are areas of extension- and subduction related tectonics, the chance that there will be volcanoes present and occasional earthquakes is highly probable. The Aegean region is thought to be moving in the southwest direction colliding into the African continental plate. This collision has created the Hellenic Trench. In fact, among geologists, the Aegean region is well known for its volcanic activity and its earthquake activity. There were two main phase of volcanism that have been publicly recognized in the Aegean region. The two volcanic belts can be separated in both time and place. The first one occurred roughly between the Oligocene to Middle Miocene period and the second phase starting in the Middle Pliocene and is still active at the present-day. In between the two phases the volcanic activity remained limited so the focus will be mainly on the two different phases.
The first phase of volcanism occurred in the north of the Aegean region and is called the North Aegean Tertiary Activity (NATA). The history of this belt is well-known as it can be traced from its creation to extinction. It also formed the basic frame-work of what we know today as the Aegean region (Andrew L. Stewart).
Subsequently the volcanic activity migrated south-westwards into what is now known as The South Aegean Volcanic Arc (SAVA), which lies parallel to the NATA and the Hellenic Trench (see figure 3). It stretches from the Greek mainland to the islands of Kos and Nisyros. The SAVA is still active today due to the northward movement of the African continental plate which leads to the subduction of the oceanic plate (Andrew L. Stewart).
The main difference between the two arcs is the thickness of the crust when the formation occurred. The NATA is believed to be formed when the Aegean plate first collided with the Oceanic plate that the African Continental plate pushed northwards. Whereas the SAVA was formed much later when the Aegean plate had undergone substantial extension. The crust is thus logically much thinner. The SAVA is thus known to have formed a back arc basin. The island of Crete is an example of a piece of continental crust that could have been split off due to extension of the Aegean plate (Andrew L. Stewart).
Figure 4: Epicentres in the Aegean region
Figure 4: Epicentres in the Aegean region
Besides volcanoes the Aegean region has also had a history of earthquakes. To determine the epicentre of an earthquake, three seismographs must be placed in the area. From these seismographs the distance to the epicentre can be calculated. If a circle is draw around each seismographic location accordingly, then the point where the three circles cross each other is the epicentre of the earthquake. The seismicity of the Aegean region is very scattered, in figure 4 we can see that the earthquakes are not homogeneously distributed, but mainly located around the Hellenic Trench and western Greece. They are related to the subduction process. The most famous fault in the Aegean region is the Northern Anatolian Fault and is located on the border of the Eurasian plate and the northern part of the Anatolian plate. An earthquake occurs when two blocks of lithospheric crust slide past each other. Three types of faults are recognized by geologists. The normal fault(1) which is when rock above the fault plane slides down the faults slope. If the rock above the fault plane is pushed up the slope this is called a reverse fault (2). Finally, if there is no up or down motion and the two blocks simply slide past each other this is called a strike and slip fault (3). The Northern Anatolian Fault has no up or down motion and is thus a strike and slip fault (Stephan Marshak).
Paleoenvironment

The paleogeographic evolution of de Mediterranean area, especially in the Atlantic gateway is crucial in order to understand the spatial distribution and sediment patterns (Krijgsman et al 2002).
One of the most dramatic episodes of change in environment regarding the Mediterranean was land-locking the sea causing the Messinian Salinity Crisis 5.96 million years ago (Krijgsman et al 2002). In the Miocene epoch (the latter part of the Messinian age), the African plate collided into the Eurasian plate for the first time making a large basin of the Mediterranean Sea. As a result, the Mediterranean went through a cycle of partially or nearly complete desiccation. This occurred when the Mediterranean Sea was no longer linked to the Atlantic Ocean. The three main passages to the Atlantic Ocean were the Beltic seaways which ran through southern Spain, the Rifian Corridor which went through northern Morocco and the Gibraltar strait which lies between Spain and Morocco. The first two were already closed before the onset of the Messinian Salinity Crisis, but the Gibraltar strait was the final cut off and transformed the Mediterranean Sea into a ‘Largo Mare’ (Govers). The theories on why and how the Gibraltar strait closed slightly differ. A short overview will be given of the different hypothesis.
The earliest causal explanation of the Messinian Salinity Crisis was that there were that extremely thick evaporites that were deposited in a deep desiccated basin of the Mediterranean area and that the area was repeatedly cut off from the Atlantic Ocean. However, the exact process of this isolation was unclear and there was never enough evidence to prove that the time frame was accurate enough to make these conclusions.
Only recently discovered technology has allowed us to accurately look into the time-frame of evaporite precipitation which has been found to be equally levelled across the Mediterranean basin at the time. Furthermore, the technology has proven that this evaporite precipitation occurred exactly 5.96 million years ago.
A more recent theory states that looks at characteristics of the sediment found that evaporite minerals, soils and fossil fuels of de deep seafloor provide evidence showing that the Gibraltar strip was complete closed off. Due to high evaporation rates the Mediterranean basin partially or almost completely dried up. As a consequence hyper saline pockets were created and at certain areas water levels fell. There is controversy on how much the water levels fell. Some scientists state that the sea level declined far under the world ocean levels (Clauzon). Whereas others believe that the water level only dropped slightly (if it dropped at all) after the massive dropping of halite. Halite has a significant effect on the isostasy (Krijgsman, 1999).
Strait uplift due to isostasy played a key role in the choking of the Mediterranean Sea. When the connection to the Atlantic ocean was lost and the sea level dropped, a sizeable uplift of the Gibraltar Strait occurred creating a large wall between the Mediterranean Sea and the Atlantic making the possibility of the area re-flooding significantly smaller (Govers).
Milankovitch cycles have also been found to have influence on the climate. They do that by affecting the global, seasonal and latitudinal distribution of solar insolation. Milankovitch cycles can be described as the periodic variations of the Earth’s orbit around the sun which is influenced by gravitational forces of the other planets and the moon. The fluctuation caused by the gravitational interruptions is found in the precession, obliquity and eccentricity. These ‘periodic cycles’ are recognized in many sedimentary records of depositional environments in the Mediterranean area. The reason why this land-locked basin is so extremely sensitive to these astronomical oscillations is mainly because of its latitudinal position and that is enclosed. This makes the sedimentary record particularly sensitive allowing accurate measurements to be made by current Earth scientists. (Krijgsman et al 2002).
Less dry conditions allowed the basin to receive more water from rivers slowly reducing de salinity. Around the same period roll-back and steepening of the Gibraltar strip allowed the Atlantic Ocean to re-enter the basin 5.33 million years ago during the Zanclean Flood (Clauzon & Govers). Today the Mediterranean Sea remains to be saltier than the Atlantic Ocean because the strait of Gibraltar remains to be very narrow while the evaporation rates are relatively high.
At present, major environmental changes of the Mediterranean Sea cannot yet be linked to tectonic or climatic changes of the Atlantic gateways. However, it seems evident that a transition from open marine marls to diatomites and evaporites are caused by the choking of one or more of the Atlantic gateways. The technology we have today allows us to accurately picture all the developments in the Mediterranean area through Satellite images. Together with Global Position Stations (GPS) this will enhance our understanding of the underlying mechanisms in the Mediterranean and let us keep track of the present day climate-change. Even though there are still many gaps and uncertainties regarding the exact past of the paleoenvironment but it is crucial to keep on investigating with the accurate technology to increase our knowledge and understanding of the geological past of the ‘Mare Nostrum’ (Krijgsman et al 2002).

References:

Andrew L. Stewart. (2003). Volcanic facies architecture and evolution of Milos, Greece. University of Tasmania (PHD).
Clauzon. (1996). Altnernate interpretation of the Messinian salinity crisis: Controversy resolved?. Geology. 24 (4), 363-366.
Rob Govers. (2009). Choking the Mediterranean to dehydration: The Messinian salinity crisis. Department of Earth Sciences, Utrecht University. - (-), -.
Stephan Marshak (2012). Earth, Portrait of a Planet. 4th ed. New York : W. W. Norton & Company. 249-328.
Wortel, W.Spakman. (2000). Subduction and Slab Detachment in the Mediterranean-Carpathian Region. Science. 290, 1910-1917
Wout Krijgsman. (2002). The Mediterranean: Mare Nostrum of Earth sciences. Earth and Planetary Science Letters. 205, 1-12.
Wout Krijsman. (1999). Chronology causes and progression of the Messinian salinity crisis. Nature. 400 (-), 652-655