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Travel log – The Kenya rift

Travel log – The Kenya rift

Topographic map of the Kenya rift and surroundings. Dark red lines indicated faults from the GEM database. Dotted blue lines separate the northern, central and southern Kenya rift. In green circles the discussed locations.

A little over a year ago, I was lucky enough to join a field trip to the Kenya rift organized by Potsdam University and Roma III. This rift is part of the active East African Rift System, which I introduced in a previous blog post. With a group of 25 enthusiastic participants from Roma Tre, Potsdam University, Nairobi University and GFZ Potsdam (we somehow always managed to make the 20-person bus work), we set out to study the interaction between tectonics, magmatism and climate and their link to human and animal evolution. Based on several pictures, I’ll take you through the highlights.

 

 

 

 

Basement foliation and fault orientation

Two numerical modellers looking at rocks… Gneisses of the Mozambique belt with steeply dipping foliation – I think. Courtesy of Corinna Kallich, Potsdam University.

Although this first picture might not look so impressive (I promise, more impressive ones will come), this road outcrop shows the structure of the basement that is responsible for the orientation of the Kenya rift’s three western border faults. Here in particular, we are slightly west of the Elgeyo escarpment, the scarp of the major east-dipping Elgeyo fault. It reactivated the steep foliation of the Mozambique belt gneisses that formed during the Pan-African orogeny (550-500 Ma; Ring 2014). Changes in foliation orientation are mirrored by changes in fault orientation from NNE to NW upon going from the Northern to the Southern Kenya rift (see map). The Elgeyo fault itself displaced the 14.5 My old massive extrusion of phonolite lavas that can be seen throughout the Kenya rift area, marking the start of the current rift phase. From the differences in basement level between the western shoulder and the rift centre, the total offset along the fault is ~4 km!

Rift axis volcanism

Lunch overlooking the Menengai caldera that collapsed 36,000 yr ago. Courtesy of Corinna Kallich, Potsdam University.

With on-going rifting, the tectonic and magmatic activity localised in the centre of the Kenya rift. One massive central volcano is the Menengai volcano, whose view we enjoyed over lunch. This 12 km wide caldera collapsed 36,000 yr ago; the ash flows of the eruption can be found throughout the whole of Kenya. Within the caldera, diatomite layers alternating with trachyte lava flows indicate the presence of lakes 12 and 5 ky ago. These lakes were fed by the neighbouring Nakuru basin overflowing into the Menengai crater. The volcano itself was responsible for the earlier compartmentalization of the larger Nakuru-Elmentaita basin. At the moment, freshwater springs are being fed by the groundwater, and 40 geothermal wells are being constructed to benefit from the groundwater being heated by the magma chamber at 3-3.5 km depth.

Lunch at Hell’s Gate

Looking along Hell’s Gate Gorge – cut into the white diatomite and pyroclastic layers – towards feeder dikes of the remaining core of a volcano. Courtesy of Corinna Kallich, Potsdam University.

Watching the wildlife and beautiful scenery is usually the reason people visit Hell’s Gate National Park, but we studied the flow structures in a highly viscous, silica-rich lava flow. We then scrambled our way through Hell’s Gate Gorge that cut into mostly diatomite lake sediments (these algae are very helpful) alternated with pyroclastic layers. Most impressive however, were the crosscut basaltic intrusions that we could trace back to the centre of an otherwise eroded volcanic dome. The well-deserved lunch was a rather frustrating affair, as Vervet monkeys took every chance at stealing our food, not even shying away from distracting us with their adorable babies.

Monkey enjoying my lunch. Courtesy of Corinna Kallich, Potsdam University.

 

 

 

 

 

Wishing the lake was back

The white diatomites of the Olorgesailie Formation, indicating the presence of a lake. Courtesy of Corinna Kallich, Potsdam University.

The Olorgesailie basin is where paleoanthropologist Louis Leakey and his wife palaeontologist Mary Leakey (Wikipedia) unearthed a score of Acheulean hand axes in the 1940s. The 600-900 ky old tools were used to dig for roots, cleave, hammer and scrape meat and can be seen in the Kariandusi museum site. Besides the hand axes (made from all the trachyte found in the area), we marvelled at the Olorgesailie Formation that contains them, which was deposited between ~1.2-0.5 Ma. The formation consists of repetitions of wetland, river and lake sediments and paleosols (fossil soils, indicating dryer conditions). As we stand baking in the sun on top of the dusty, white diatomite, the vision of a lake sure is very alluring.

A not-so-fresh lake

On our way to a tiny hotspring along the edge of the slightly pink waters of Lake Magadi. In the foreground the white evaporates the lake is mined for. Courtesy of Corinna Kallich, Potsdam University.

While we mostly stayed in resorts, our only campsite (proper “glamping” with a shower and bathroom in the tent) was close to Lake Magadi, one of the lakes along the rift axis. This saline, alkaline lake gave its name to magadiite, a sodium hydro silicate, that when dehydrated forms chert (i.e. flint). The lake is also mined for its sodium carbonate, known as trona. During the African Humid Period (15,000-5,000 yr ago; Maslin et al. 2014), Lake Magadi was about 40 m higher, a lot fresher and connected to Lake Natron further south. Fun fact from Wikipedia: elephants visit the Magadi Basin to fill up on their own salts supplies as well. From my own experience, I can tell you, it does not taste very good.

 

 

My trusted companions for over a decade did not survive Kenya’s heat and volcanics… Serves me right for not taking them out often enough!

And then there were the hippos, neptunic dikes, dancing Maasai, a boat trip to the hydrothermal vents on Ol Kokwe Island, giraffes outside our cabin, midnight stargazing… too much to capture in one blog post. I had a wonderful time in Kenya exploring the geology, admiring the wildlife and getting to know its people. My only regret? Losing my shoes…

 

 

 

 

References:

Maslin, M. A., Brierly, C. M., Milner, A. M., Shultz, S., Trauth, M. H., Wilson, K. E. (2014). East African climate pulses and early human evolution, Quaternary Science Reviews 101, 1-17.

Ring, U. (2014). The East African Rift System, Austrian Journal of Earth Sciences, 107, 1.

Strecker, M. R., Faccenna, C., Wichura, H., Ballato, P., Olaka, L. A. and Riedl, S. (2018). Tectonics, seismicity, magmatic and sedimentary processes of the East African Rift Valley, Kenya, Kenya Field School Field Guide.

Personal communication with Strecker, M. R., Wichura, H., Olaka, L. A. and Riedl, S.

Remarkable Regions – The Kenya Rift

Remarkable Regions – The Kenya Rift

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. After looking at several convergent plate boundaries, this week the focus lies on part of a nascent divergent plate boundary: the Kenya Rift. The post is by postdoctoral researcher Anne Glerum of GFZ Potsdam.

Of course an active continental rift is worthy of the title “Remarkable Region”. And naturally I consider my own research area highly interesting. But after seeing it up-close and personal on a recent 10-day trip organized by the University of Potsdam, Roma Tre and the University of Nairobi (stay tuned for the travel log, or read that of the University of Potsdam), I must say, the Kenya Rift is a truly beautiful and fascinating region.

Figure 1. Topography (Amante and Eakins 2009) and kinematic plate boundaries (Sarah D. Stamps based on Bird 2003) of the East African Rift System (EARS). Plate boundary colors schematically indicate the western and eastern branches of the EARS.

Constituting one segment of the 5000 km long East African Rift System (EARS, Fig. 1), the Kenya Rift is host to an amazing landscape, wildlife and people, all of which somehow tie back to continental rifting processes. Although the youngest rifting phase in Kenya commenced in the Miocene, the east African region as a whole has been shaped by rifting episodes since Permian times (Bosworth and Morley 1994). The present active rift system runs from the Afar region in the north all the way south to Mozambique and is split into a western and an eastern branch that run around the Archean Tanzanian Craton (Chorowitz 2005, see Fig. 1). Generally speaking, the western branch is more seismically active, but deprived of magmatism, compared to the eastern branch, of which the Kenya Rift is part (Chorowitz 2005). Three processes characterize the EARS (Burke 1996) as well as the Kenya Rift specifically: normal faulting, volcanism and uplift.

Uplift

The Tanzanian Craton together with the enveloping western and eastern EARS branches constitutes the broad, uplifted area coined the East African Plateau (~1200 m elevation, Strecker 1991; Simiyu and Keller 1997, Fig. 2). The onset of uplift of this plateau can be constrained to the Early Miocene with the help of one of the longest phonolitic lava flows on Earth (> 300 km, Wichura et al. 2010; 2011) and a whale that stranded inland 17 Ma (and was only recently found again after going missing for 30 years, Wichura et al. 2015). Plume-lithosphere interaction is thought responsible for the uplift (e.g. Wichura et al. 2010), although there is disagreement about the continuity of the low seismic velocity anomalies seen in the east African upper mantle and whether they are connected to the lower mantle. For example Ebinger and Sleep (1998), Hansen et al. (2012), Sun et al. (2017) and Torres Acosta et al. (2015) advocate for one East African superplume, while Pik et al. (2006) distinguish separate lower and upper mantle plumes and Davis and Sack (2002) and Halldórsson et al. (2014) consider a lower mantle plume splitting in the upper mantle.

Figure 2. Topography (Amante and Eakins 2009) and fault traces (GEM) of the central EARS. Triangles indicate off-rift volcanoes, dotted grey lines the three segments of the Kenya Rift.

Magmatism and volcanism

The northward motion of Africa over this hot mantle anomaly has been thought the cause of a north-to-south younging trend in the age of the ensuing EARS volcanism and rifting (e.g. Ebinger and Sleep 1998; George et al. 1998; Nyblade and Brazier 2002), although more recent studies arrive at a more spatially disparate and diachronous rifting evolution (Torres Acosta et al. 2015 and references therein). In general, massive emplacement of flood-phonolites preceded the onset of rifting in Kenya around 15 Ma (Torres Acosta et al. 2015). With ongoing rifting, and localization of faulting towards the rift axis, volcanism also migrated towards the center of the rift. Since the Miocene, massive amounts of volcanics have thus been emplaced (144,000-230,000 km3, MacDonald 1994; Wichura et al. 2011). Moreover, dyking also accommodated a significant part of the extension, with 22 to 26 % of the crust in the rift valley being composed of dykes (MacDonald 2012). Not surprisingly, the highlands directly around the rift valley, the Kenya Dome (Fig. 2) formed through a combination of volcanism and uplift (Davis and Slack 2002) with elevations of up to 1900 m.

The composition of rift magmatism is bimodal, showing phonolites and trachytes on the one side and nephelinites and basalt on the other, predominantly resulting from fractional crystallization of a basaltic source. The low viscosity of these magmas allows the young volcanoes in the volcano-tectonic axis to reach significant heights (see Fig. 3; MacDonald 2012). The most impressive volcanoes are to be found outside of the rift however (Fig. 2), with Mnt. Elgon reaching 4321 m and Africa’s highest mountains Mnt. Kenya and Mnt. Kilimanjaro reaching up to 5200 m and 5964 m, respectively (Chorowitz 2005).

Figure 3. View on the crater rim of the 400 ky old Mnt. Longonot volcano in the tectono-magmatic rift axis, at 2560 m asl. Courtesy of Corinna Kallich, GFZ Potsdam.

Normal faulting

The Kenya rift itself is composed of 3 asymmetric segments, distinguished by sharp changes in their orientation (Chorowitz 2005, Fig. 2). The 2300-3000 m high Elgeyo, Mau and Nguruman escarpments result from the steep Miocene east-dipping border faults in the west, while the antithetic border faults on the eastern side formed later during the Pliocene (Strecker et al. 1990). The older border faults formed along preexisting foliation generated by the Mozambique Belt orogeny in the late Proterozoic (Shackleton 1993; Hetzel and Strecker 1994). A change in strike of this foliation from NNE in the northern and southern Kenya rifts to NW determined the change in orientation in the central Kenya rift (Strecker et al. 1990). Consequently, different generations of faults in the northern and southern rift segments run parallel, while in the central segment, the Pleistocene change in extension direction from ENE-WSW/E-W to the present-day WNW-ESE/NW-SE directed extension results in obliquely reactivated border faults and younger, en echelon arranged left-stepping NNE-striking fault zones along the rift axis (Strecker et al. 1990). Extension is transferred between the different zones by coeval normal and strike-slip faulting or dense sets of normal faults.

Figure 4. View of lake Magadi and the Nguruman escarpment. Lake Magadi is a saline, alkaline lake, commercially mined for trona. Courtesy of Corinna Kallich, GFZ Potsdam.

Human evolution

The uplift, volcanism and normal faulting together have set the stage for human and animal evolution. For example, the shift in hoofed mammals from eating predominantly woods to grazing species evidences that the large-scale uplift modified air circulation patterns resulting in aridification and savannah-expansion at the expense of forested areas (Sepulchre et al. 2006; Wichura et al. 2015). The rift basins enabled the formation of large lakes, which were subsequently compartmentalized by tectonic and volcanic morphological barriers (Fig. 4). On the short-term, lake coverage varied due to tectonically induced changes in catchment areas, drainage networks and outlets. Maslin et al. (2014) actually found a correlation between this ephemeral lake coverage and hominin diversity and dispersal. Lake highstands link with the emergence of new species and allowed the spread of hominins north and southward out of east Africa. Remarkable, or what!

References:
Amante, C. and Eakins B. W., 2009. NOAA Technical Memorandum NESDIS NGDC-24. National Geophysical Data Center, NOAA.
Bosworth, W. and Morley, C.K., 1994.  Tectonophysics 236, 93–115.
Burke, K., 1996. S. Afr. J. Geol. 99 (4), 339–409.
Chorowitz, J., 2005. J. Afr. Earth Sci. 43, 379-410.
Davis, P. M. and Slack, P. D. 2002. Geophys. Res. Lett. 29 (7), 1117.
Ebinger, C.J. and Sleep, N.H., 1998. Nature 395, 788-791.
George, R. et al., 1998.  Geology 26, 923–926.
Halldórsson, S. A. et al., 2014. Geophys. Res. Lett. 41, 2304–2311,
Hansen, S. E. et al., 2012.  Earth Planet. Sc. Lett. 319-320, 23-34.
Hetzel, R., Strecker, M.R., 1994. J. Struct. Geol. 16, 189–201.
Macdonald, R. et al., 1994a. J. Volcanol. Geoth. Res. 60, 301–325.
Macdonald, R., et al., 1994b. J. Geol. Soc. London 151, 879–888.
MacDonald, R., 2012. Lithos 152, 11-22.
Maslin, M. A. et al., 2014. Quaternary Sci. Rev. 101, 1-17.
Nyblade, A. A. and Brazier, R. A., 2002. Geology 30 (8), 755-758.
Pik, R. et al., 2006. Chem. Geol. 266, 100-114.
Sepulchre, P. et al., 2006. Science, 1419-1423.
Shackleton, R.M., 1993. Geological Society, London, Special Publications 76, 345–362.
Simiyu, S.M., Keller, G.R., 1997. Tectonophysics 278, 291–313.
Strecker, M., 1991. Das zentrale und südliche Kenia-rift unter besonderer berücksichtigung der neotektonischen entwicklung, habilitation, Universität Fridericiana.
Sun, M. et al., 2017.  Geophys. Res. Lett. 44, 12,116–12,124.
Torres Acosta, V. et al., 2015. Tectonics 34, 2367–2386.
Wichura, H. et al., 2010. Geology 38 (6), 543–546.
Wichura, H. et al , 2011. The Formation and Evolution of Africa: A Synopsis of 3.8 Ga of Earth History, eds. D. J. J. Van Hinsbergen, S. J. H. Buiter, T. H. Torsvik, C. and Gaina, S. J.
Wichura, H. et al., 2015. P. Natl. Acad. Sci. USA 112 (13), 3910-3915.

Alaska: a gold rush of along strike variations

Alaska:  a gold rush of along strike variations

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. After exploring the Mediterranean and the ancient Tethys realm, we now move further north and across the Pacific to the Aleutian-Alaska subduction zone. This post was contributed by Kirstie Haynie who is a PhD candidate at the department of geology at the University at Buffalo, State University of New York, in the United States of America.

Given that Alaska is a remarkable region, I decided to walk up to strangers and ask them what comes to mind when they hear the word “Alaska”. Indeed I received some confusing looks and laughs, but everyone I asked had something to say. Some people alluded to popular TV shows set in Alaska, such as Gold Rush, Bush People, and Alaska: the Last Frontier, while others spoke about the cold weather, dog mushing, Eskimos, fishing and hunting, and the Trans-Alaska pipeline. A few of the answers I received referenced the beauty and wilderness of the large snow capped mountains, glaciers, and the Northern Lights (Aurora Borealis): all emblematic of the largest state in America. But to me, Alaska is more than just a pretty landscape and a place to fish. It is a region riddled with geologic mysteries and rich in along strike variations.

The Aleutian-Alaska subducton zone marks a North American-Pacific plate boundary where subduction varies greatly along strike (Figure 1). At the western end of the subduction zone, the Aleutian volcanic islands are the result of oceanic-oceanic subduction while in the eastern part of the subduction zone there is oceanic-continental collision where the Pacific plate descends beneath the North American plate. The age of the subducting sea floor increases laterally from around 30 Ma in the eastern subduction corner to 80 Ma at the end of the Aleutian volcanic arc (Müller et al., 2008). Slab dip changes drastically from 50° to 60° in the west and central Aleutians to flat slab subduction under south-central Alaska (Ratchkovski and Hansen, 2002a; Lallemand et al., 2005; Jadamec and Billen, 2010). This leads to a variation in the slab pull force, which is a main driving force of subduction caused by the weight of dense slabs sinking into the mantle (Morra et al., 2006).

Figure 1: Tectonic map of Alaska modified from Haynie and Jadamec (2017). Topography/bathymetry is from Smith and Sandwell (1997) and Seafloor (SF) ages are from Müller et al. (2008). Blue lines are the slab contours of Jadamec and Billen (2010) in 40 km intervals; the thick black line is the plate boundary from Bird (2003); and the thinner black lines are faults from Plafker et al. (1994a). The location of Denali is marked by the orange hexagon. Holocene volcanoes are given by the pink triangles (Alaska Volcano Observatory). The purple polygon is the outline of the Yakutat oceanic plateau (Haynie and Jadamec, 2017). WB – Wrangell block fore-arc sliver; JdFR – Juan de Fuca Ridge.

There is also a distinct change in margin curvature from convex in the west to concave in the east. At the end of the eastern bend, the Alaska part of the subduction zone is truncated by a large transform boundary, the Fairweather-Queen Charolette fault, which gives rise to a corner-shaped subduction-transform plate boundary (Jadamec et al., 2013; Haynie and Jadamec, 2017). Here, convergence is oblique with an average velocity of 5.2 cm/year northwest (DeMets and Dixon, 1999). Seismic studies (Page et al., 1989; Ferris et al., 2003; Eberhart-Phillips et al., 2006; Fuis et al., 2008) show that thicker than normal oceanic crust lies off-shore in the subduction corner. This thick oceanic material has been identified as the Yakutat oceanic plateau (Plafker et al., 1994a; Brocher et al., 1994; Bruns, 1983; Worthington et al., 2008; Christeson et al., 2010; Worthington et al., 2012). Even though oceanic plateaus tend to resist subduction (Cloos, 1993; Kerr , 2003), the Yakutat plateau is currently subducting beneath the Central Alaska Range to depths of 150 km (Ferris et al., 2003; Eberhart-Phillips et al., 2006; Wang and Tape, 2014). It is also colliding into south-east Alaska (Mazzotti and Hyndman, 2002; Elliott et al., 2013; Marechal et al., 2015) where the largest coastal mountain range on Earth, the Saint Elias Mountains, are located (Enkelmann et al., 2015).

With regards to surface deformation, in addition to Denali (the tallest mountain in North America), other notable along strike variations reside within the broad deformation zone of south-central Alaska. For example, a normal volcanic arc occurs over the Aleutian part of the subduction zone and above the Alaska Peninsula. However, above the flat slab there is a gap in volcanism followed by the presence of the enigmatic Wrangell volcanoes (Rondenay et al., 2010; Jadamec and Billen, 2012; Martin-Short et al., 2016; Chuang et al., 2017). These volcanoes are marked by a range of morphologies as well as adakitic geochemical signatures (Richter et al., 1990; Preece and Hart , 2004), which have a petrogenesis that may be attributed to slab melting (Defant and Drummond , 1990; Peacock et al., 1994; Castillo, 2006, 2012; Ribeiro et al., 2016). Analogue (Schellart , 2004; Strak and Schellart , 2014) and 3D numerical models (Stegman et al., 2006; Piromallo et al., 2006; Jadamec and Billen, 2010, 2012) predict that toroidal flow can produce upwellings around the edge of a slab that may have implications for melting of the slab and the formation of adakites. However, the formation of the Wrangell volcanoes is still debated.

Also located above the subducting plateau and flat slab is the Wrangell block fore-arc sliver, which exhibits northwest motion and counterclockwise rotation (Cross and Freymueller, 2008; Freymueller et al., 2008; Bemis et al., 2015; Waldien et al., 2015; Jadamec et al., 2013; Haynie and Jadamec, 2017). This sliver is bounded in the north by the arcuate shaped Denali fault, which illustrates a lateral change in slip rates that increases towards the center of the fault (Haynie and Jadamec, 2017; Haeussler et al., 2017). 3D high-resolution geodynamic models show that the flat slab drives motion of the Wrangell block fore-arc sliver (Jadamec et al., 2013; Haynie and Jadamec, 2017) and contributes to fault parallel motion along the eastern Denali fault and convergence along the apex of the fault (Haynie and Jadamec, 2017) (Figure 2). However, when model predictions of the Wrangell block motion and the difference in Denali fault parallel motion are compared with observations, model predictions are lower, suggesting that the flat slab alone is not sufficient enough to explain the broad deformation zone of Alaska (Haynie and Jadamec, 2017). Thus, it is thought that the neotectonics of south-central Alaska are predominantly driven by the subduction-collision of the buoyant Yakutat oceanic plateau (Bird , 1988; Plafker et al., 1994b; Fitzgerald et al., 1995; Ratchkovski and Hansen, 2002b; Bemis and Wallace, 2007; Chapman et al., 2008; Haeussler , 2008; Jadamec et al., 2013; Lease et al., 2016; Haynie and Jadamec, 2017). 4D numerical modelling of this process is currently underway.

Figure 2: Top: map of south-central Alaska (zoomed in from Figure 1) with model predicted velocities (blue arrows) from Haynie and Jadamec (2017) plotted on top. Bottom: percent of slab contribution from Haynie and Jadamec (2017) models to observed Denali fault slip rates (modified from Haynie and Jadamec (2017)). Results from Haynie and Jadamec (2017) show that the slab drives northwest and counter-clockwise motion of the Wrangell block fore-arc sliver and contributes to an average of 20-28% of motion along the Denali fault. The flat slab exerts the largest contribution to motion along the eastern segment of the fault, where surface motion parallels the fault, and also along the central segment of the fault, where the slab is driving the Wrangell block into the North American backstop and subducting obliquely to the fault.

 

References
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Bemis, S. P., R. J. Weldon, and G. A. Carver (2015), Slip partitioning along a continuously curved fault: Quaternary geologic controls on Denali fault system slip partitioning, growth of the Alaska Range, and the tectonics of south-central Alaska, Lithosphere, 7 (3), 235–246.
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The eastern Mediterranean: What’s in a name?

The eastern Mediterranean: What’s in a name?

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. To kick off this series, Anne Glerum introduces us to the eastern Mediterranean, which has been a natural laboratory for generations of scientists.

The name of our Remarkable Region is quite descriptive: it designates the region around and including the eastern part of the Mediterranean Sea. From the Latin word mediterraneus, meaning in the middle of land (Wikipedia), this Sea is a large body of water surrounded by land: the African, European and Asian continents. In turn, the convergence of these continents is what helped shape the region. Such a meeting of continents is in itself a promise of scientific treasure.

McKenzie phrased the cause of scientific interest in Mediterranean deformation a little more prosaically in 1972: “it is an accessible and reasonably well-studied area where the motion between the major plates involved is well known”. These reasons have only become more valid today. The first point will be readily agreed upon; perhaps you are even reading this blog post while stretched out on one of the Mediterranean’s beautiful beaches (or, more in line with my view of geo-people on vacation, after a week-long hike along the tops of an Alpine mountain chain).

McKenzie’s second point is related to both the first and the last: the more readily accessible a region is, the more easily data can be collected and hypotheses tested. At the same time, knowledge of the major plate motions provides boundary conditions to the region under investigation. The major plates involved in the Mediterranean (Fig. 1) are the Nubian and Arabian plates presently converging at about 0.6 and 1.5 cm/yr, respectively, with Eurasia (Nocquet 2012).

The interaction of these major plates was part of the evolution of the larger Tethys region with its Alpine-Himalayan orogenic belt now running from the Mediterranean to Indonesia (Hafkenscheid 2004). The Tethys region is named after the Proto-, Paleo- and Neo-Tethys oceanic domains (Berra and Angiolini, 2014), whose opening and closing resulted in the continental collisions forming this mountain chain. For a clearer mental picture, watch for example these reconstructions by Zahirovic et al. 2012, 2016.

Figure 1: The Mediterranean region and its three major plates (Nubia, Arabia and Eurasia). Brown shaded areas indicate Alpine fold and thrust belts. Plate motions are indicated w.r.t. Eurasia by black arrows. Image credit: Modified by Anne Glerum from Woudloper (Own work) [CC BY-SA 1.0 (http://creativecommons.org/licenses/by-sa/1.0)], via Wikimedia Commons.

Consumption of the Tethyan oceanic domains occurred through mostly northward subduction underneath Eurasia (the northern Nubia-Arabia margin is a passive margin). Back-arc spreading related to roll-back of the subducting plates created smaller oceanic basins; the subsequent closure of the smaller and larger basins resulted in the accretion of continental fragments to Eurasia (Hafkenscheid 2004).

Within our remarkable region, the approximately Oligocene closure of the Neo-Tethys (e.g. Hafkenscheid 2004; Agard et al. 2011; Berra and Angiolini 2014) resulted in the continental collision of Arabia and Eurasia. Remnants of this Neo-Tethys subduction are the Bitlis and Zagros suture zones (Hafkenscheid 2004). To the west, in the Aegean region, the Nubian plate is still subducting, as it has been continuously for at least the last 100 My (e.g. van Hinsbergen et al. 2005; Jolivet and Brun 2010). This continuous subduction included oceanic domains as well as continental fragments of about 300-500 km (Facenna et al. 2003; van Hinsbergen et al. 2005; Jolivet and Brun 2010), of which the upper crust was scraped off and accreted as nappe stacks (van Hinsbergen et al. 2005).

While these nappe stacks are mostly preserved on mainland Greece (Jolivet and Brun 2010), back-arc extension has thinned the Aegean-west Anatolia region (van Hinsbergen and Schmid 2012; Faccenna et al. 2014; Menant et al. 2016;) after the Paleocene compressional phase that resulted in a.o. the Dinarides and Hellenides mountain belts (Faccenna et al. 2014, see Fig. 1). Due to the slab-retreat related extension, high-temperature methamorphic domes were exhumed (van Hinsbergen and Schmid 2012; Facenna et al. 2014). The speed of extension of the Aegean-west Anatolian region increased significantly around 15 Ma (Faccenna et al. 2003; van Hinsbergen and Schmid 2012; Menant et al. 2016), coincident with a bending of the subduction zone, possibly facilitated by tearing of the Aegean slab below western Anatolia (Jolivet et al. 2015).

At the present-day, Nubian subduction and trench retreat are still ongoing. GPS velocity fields illustrate how the motion of the Aegean and Anatolian plates differs from the overall Nubia-Eurasia convergence: their counter-clockwise rotation is facilitated by the strike-slip North Anatolian Fault and Trough and the East Anatolian Fault and increases towards the Hellenic trench (Le Pichon and Kreemer 2010; Nocquet 2012). These motions result from the interplay –in various proportions according to different authors- of the continental escape of Anatolia, Hellenic trench retreat, gravitational potential energy variations and asthenospheric flow (e.g. Le Pichon and Kreemer 2010; Faccenna and Becker 2010; England et al. 2016; Menant et al. 2016).

All in all, the present eastern Mediterranean has a complex geological history that has sparked and continues to spark the interest of many geo-scientists. Faccenna et al. (2014) neatly summarize the new concepts that were coined and/or tested based on the accessibility, wealth of data and known boundary conditions of the Mediterranean region, such as oroclinal bending and the opening of back-arc basins, extensional and strike-slip tectonics in an overall convergent setting, continental escape, trench rollback and slab tearing. A remarkable region indeed!

References:

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