Jannis Makris

The Afar Depression: transition between continental rifting and sea-floor spreading

Abstract Detailed geophysical and geological studies were undertaken in the 1970s in the Afar Depression, Ethiopia, in order to study the processes of continental break-up occurring in this unique setting; the Afar is located between the Ethiopian and Somali Plateaus and the Red Sea. We have re-evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic, gravity, seismicity and seismic refraction data. The results of the re-evaluation indicate that the Afar Depression is a transition zone between the continental rifts of Kenya and the present ongoing sea-floor spreading of the Red Sea and the Gulf of Aden. In the Depression itself the crust is greatly attenuated and is underlain by a low velocity (7.4–7.5 km/s), high temperature upper mantle material. However, it is still a continental crust which thins to the northeast and east towards the Red Sea and the Gulf of Aden. It is not a newly formed oceanic crust as has been suggested in the past. The trends of the areas of crustal thinning are indicated by the gravity anomalies. The trend of the Ethiopian Rift reaches South Afar where the trend of crustal thinning changes and becomes parallel to the Red Sea trend. It is offset en echelon from the spreading area of the Red Sea, which is marked by an alignment of gravity maxima. The Gulf of Aden trend continues west towards the Ethiopian Rift. The gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development.

The crust and upper mantle of the Aegean region from deep seismic soundings

Abstract Deep seismic soundings performed in Greece in the years 1971–1974 revealed that the area of Greece consists of continental crust of variable thickness. The crust of the west Hellenic chains is 46 km thick and covered by sediments which locally exceed 10 km. That of the Aegean Sea is attenuated from north to south having 32 km below north Evia, 30 below south Evia and 28 km below Mikonos. The Cretan Sea shows an east-west elongated dome with maximum crustal attenuation at the central trough zone, with only 20 km thickness. The crust of Crete increases again to 32 km. Good Pg, Pn and PmP seismic phases were recorded at nearly all observed sections. They showed that the basement has a true velocity of 6 km/sec and that the lower crust is controlled by a layer of gradual velocity increase from about 6.2 to 6.8 km/sec. The Pn velocities have smaller values than the normal upper mantle has, ranging between 7.6 and 7.8 km/sec and indicating that below the Hellenides and the Aegean Sea a low velocity zone exists in the uppermost part of the mantle, limited in depth by a high velocity layer of approx. 8.5 km/sec. The geometry of the crust—mantle boundary across the coast of Peloponnesos showed that the crust nearly doubles in thickness from 26 to 46 km in only 10 km of horizontal distance.

Crustal structure of the Levant Basin, eastern Mediterranean

Abstract A seismic refraction/wide-angle reflection experiment was undertaken in the Levant Basin, eastern Mediterranean. Two roughly east–west profiles extend from the continental shelf of Israel toward the Levant Basin. The northern profile crosses the Eratosthenes Seamount and the southern profile crosses several distinct magnetic anomalies. The marine operation used 16 ocean bottom seismometers deployed along the profiles with an air gun array and explosive charges as energy sources. The results of this study strongly suggest the existence of oceanic crust under portions of the Levant Basin and continental crust under the Eratosthenes Seamount. The seismic refraction data also indicate a large sedimentary sequence, 10–14 km thick, in the Levant Basin and below the Levant continental margin. Assuming the crust is of Cretaceous age, this gives a fairly high sedimentation rate. The sequence can be divided into several units. A prominent unit is the 4.2 km/s layer, which is probably composed of the Messinian evaporites. Overlying the evaporitic layer are layers composed of Plio–Pleistocene sediments, whose velocity is 2.0 km/s. The refraction profiles and gravity and magnetic models indicate that a transition from a two layer continental to a single-layer oceanic crust takes place along the Levant margin. The transition in the structure along the southern profile is located beyond the continental margin and it is quite gradual. The northern profile, north of the Carmel structure, presents a different structure. The continental crust is much thinner there and the transition in the crustal structure is more rapid. The crustal thinning begins under western Galilee and terminates at the continental slope. The results of the present study indicate that the Levant Basin is composed of distinct crustal units and that the Levant continental margin is divided into at least two provinces of different crustal structure.

Lateral variation of the crust in the Iberian peninsula: New evidence from the Betic Cordillera

Abstract New results from a seismic refraction/wide-angle reflection survey carried out in the Betic Cordillera in autumn 1989, contribute to a better picture of its deep structure. One NW-SE profile cuts across the Iberian Massif, the external and internal Betics. The structure of the crust in the Iberian Massif shows characteristics similar to those found in previous experiments. The lower crust is found as a distinct layer, 12 km thick, with an average velocity of 6.8 km · s−1; the Moho is found at about 35 km depth. This structure extends southeastward until a 3–4 km upwelling of the Moho, about 30 km north of the present-day surface boundary between the external and internal Betic units. Further southeast the Moho deepens to 38 km and the lower crust is no longer seismically detected. The absence of differentiated lower crust beneath part of the external Betics may be related with the Mesozoic rifting of the South-Iberian passive margin. Instead, this absence under the internal Betics may be caused by rifting in conjunction with the collisional evolution of the orogen. A WNW-ESE profile lying in the internal Betics shows the presence of a prominent reflector at 10–12 km depth. This seems to be a widespread feature in the internal Betics and may be interpreted as a detachment surface. The Moho is found at 38 km depth rising strongly in the easternmost Betics. Seismic data suggest a thin crust in the offshore area southwest of Malaga, probably containing a massive zone of high-velocity rocks which is also supported by available geophysical and geological data.

A crustal structure study of Jordan derived from seismic refraction data

Abstract The interpretation of a deep seismic refraction study in Jordan, performed in May 1984, shows that much of the country is underlain by continental crust, 32–35 km thick, and normal mantle with a velocity of 8.0–8.2 km/s. In the Aqaba region, southwest and central Jordan, east of Wadi Araba, the crustal thickness is of the order of 32–35 km, while in the Amman region it is not less than 35 km. In southeast Jordan the crust thickens to at least 37 km in what is probably the transition to the Arabian Shield type of crust. The boundaries between the upper and lower crust at about 20 km depth and the lower crust and uppermost mantle are probably transition zones. The upper crystalline crust has velocities of 5.8–6.5 km/s while the lower crust has velocities greater than or around 6.65 km/s. While the crystalline basement is exposed in southwest Jordan and is at a depth of 2–2.5 km north of Amman, it is at a depth of not less than 5 km in central Jordan. A comparison of the crustal type and structure of Jordan and the adjacent Dead Sea rift with that of the Black Forest and the Rhine valley yields a striking resemblance. The situation of the Jordan-Dead Sea rift is explained in terms of the continental crust of Arabia rifting in preference to the thin (?oceanic) crust of the Mediterranean Sea.

Physical properties and state of the crust and upper mantle of the Eastern Mediterranean Sea deduced from geophysical data

Abstract By evaluating the geophysical information available for the Eastern Mediterranean Sea (EMS), a crustal thickness map and some conclusions as to the nature of the crustal type were drawn. It was found that the Ionian Abyssal Plain is floored by an oceanic type of crust which is 14 km thick and covered by sediments of the order of 8 km thick. The Herodotus Abyssal Plain in the Levantine Sea is floored by a crust 22 km thick and covered by sediments ranging between 10 and 15 km thick. The Eastern Mediterranean Rise, extending more or less continuously from the Apulian Plateau to Rhodes and the south of the Antalya Basin, is of a thickness ranging between 28 and 34 km. The thickness of its sedimentary cover is of the order of 10 km. The borders of the Eastern Mediterranean Sea are built up of thick continental crust. For example the Hellenides of Western Greece range between 40 and 44 km thick and Western Turkey has a similar crustal thickness. The crust of Sicily also exceeds 40 km whereas the southern and eastern borders of the EMS are approximately 30–32 km thick. Tectonically the northern border of the EMS is built up of active continental margins defined by the Calabrian, Hellenic and Cyprus—Antalya Arcs, whereas the southern border represents a passive continental margin controlled mainly by subsidence and stretching. To the east, the Dead Sea Megashear limits the area. The deep basins of the Eastern Mediterranean Sea are tectonically controlled by subsidence and partial stretching. The Mediterranean Ridge or Rise is still an area of dispute, regarded by some authors as a subsiding continental fragment and separated from the Hellenic Arc by the Hellenic Trench, whereas others consider it to be a zone of crustal shortening, thickened mainly by thrusting. Both the western and southwestern borders of the Mediterranean Rise and the Hellenides are controlled by compressional processes similar to those deforming the Calabrian Arc. The Stravo and Pliny trenches to the east-southeast border of the Hellenic Arc are partly affected by strike-slip movements. Various plate-tectonic models of the area have been proposed by several authors, though the complexity of the deformation hardly permits reliable reconstructions. The lack of reliable paleomagnetic information and the absence of linear magnetic anomalies do not favour the application of plate tectonic concepts. Vertical processes due to the uprising of lithothermal systems below the Hellenic and Calabrian arcs may also be regarded as the driving force controlling the present-day deformation.

Crustal investigation of the Hellenic subduction zone using wide aperture seismic data

Abstract We present the results of a wide-aperture seismic onshore–offshore study (Crete Seismic Experiment) in the Cretan region as part of the Hellenic arc compressional system. Three seismic lines were carried out on and around the island of Crete in order to investigate the crustal structure of the region. Up to 119 three-component recording stations were deployed on each profile that observed seismic energy generated by a 48-l airgun array and eight 20-kg landshots. A total of 6208 shots were fired. Upon completing the fieldwork, the vertical components of all stations were evaluated; 300 Common-Receiver-Gather (CRG) sections of the ocean bottom seismographs (OBS) and land stations as well as 100 Common-Source-Gather (CSG) sections of the land shots and selected airgun shots were compiled and modeled in order to generate a 2D P-wave velocity–depth model for each profile. The accuracy of the model depends on the depth and position along the profiles and does not exceed 5% for both depth and P-wave velocity. We identified strong lateral variations in crustal and sedimentary thickness mainly in a north–south direction but also along strike (east–west). The crust is continental and has a maximum thickness of 32.5 km below northern central Crete. Its subdivision in an upper ( v p =5.8–6.3 km/s, locally up to 6.5 km/s) and a lower ( v p =6.4–6.9 km/s) part is justified by a first-order discontinuity with v p -velocity a contrast of up to 0.6 km/s. The eastern part of Crete shows a significantly thinner crust of 24 to 26 km. To the North, the crustal thickness decreases to 15 km below the central Cretan Sea. The prominent decrease of the Moho depth north of central Crete is interpreted to represent the northern end of a microcontinent that was subducted in Oligocene times and later surfaced by ‘buoyant escape’ (Stockhert et al., 1999; Thomson et al., 1999). The P – T – t – D history of the high-pressure rocks of Crete, Greece: denudation by buoyant escape. In: Exhumation Processes: Normal Faulting, Ductile Flow and Erosion. Ring, U., Lister, G., Willet, S., Brandon, M. (Eds.), Spec. Publ. of the Geol. Soc. of London, p. 154]. To the south and southwest of the island, the continental crust gradually thins to a minimum of 17 km and at approximately 100 km off the southern coast of Crete, it is in contact with oceanic crust below the Mediterranean Ridge. Upper mantle velocities were determined to be 7.7 km/s below the Cretan Sea and 8.0 km/s south of Crete. Below the continental Cretan crust, a 6- to 7-km-thick layer with v p -velocities between 6.6 and 7.1 km/s was identified on each line and could be followed by reflections to a depth of 42 km. It is decoupled from the overlying continental crust at central Crete and is interpreted as oceanic crust presently under subduction towards the NNE below the Aegean Sea.

Shear-controlled evolution of the Red Sea: pull apart model

Abstract Results of seismic and other geophysical investigations suggest that strike-slip processes controlled the break-up of the Arabian plate from Africa and initiated the Red Sea Rift. Early oceanisation was facilitated by nucleation of pull apart basins and massive intrusives. The evolution of the Red Sea has gone through different stages. It was a zone of structural weakness already during the Pan-African orogeny approximately 600 Ma. A major reactivation, however, that gradually led to the present-day configuration was initiated during the late Oligocene with intense magmatic activity and the development of a continental rift. Wrench faulting played a key role in the early evolution of the Red Sea, as it shaped most of its western flank as a sharp plate boundary and resulted in the generation and rapid oceanisation of linearly arranged pull apart basins. Spatial distribution of these basins reflects the geometry of the strike-slip zone, which was controlled by pre-existing fault systems like the Najd Shear System, the Central African Fault Zone or the Onib-Hamisana and Baraka suture zones. Strike-slip motion along the latter zones of weakness influenced mainly the Egyptian and Sudanese coastal areas. Arabia was therefore separated from Africa by oceanisation in those regions, where pull apart basins developed. They were still connected in the in-between segments by stretched continental crust. With Arabia as the “moving” and Africa as the “stable” plate the eastern Red Sea flank was formed by pure shear through stretching, thinning and diffuse extension. As a consequence, the eastern and western flanks of the Red Sea are asymmetrical. The acceleration of the movement of Arabia in early/middle Miocene could no longer be accommodated by the opening in the Gulf of Suez and consequently the Dead Sea strike-slip fault developed approximately 14 Ma ago. Since plate motion was still oblique to the major structural trends, the pull apart evolution on the western flank continued and the oceanisation was accentuated, while in the north the eastern flank developed en-echelon fractures of “Aqaba-orientation”. Seafloor spreading commenced 5 Ma ago at parts of the central and southern Red Sea. This final stage is also responsible for the bathymetry with maximum depth values of 3000 m in the seafloor spread areas and a much shallower basin in the north. The metalliferous “Red Sea deeps” are either associated with this infant mid-ocean ridge or with the en-echelon fractures in the north.

A dynamic model of the Hellenic Arc deduced from geophysical data

Abstract Combined gravity and seismic data from Greece and the adjacent areas have been used to explain the high seismicity and tectonic activity of this area. Computed 2-D gravity models revealed that below the Aegean region a large “plume” of hot upper-mantle material is rising, causing strong attenuation of the crust. The hot “plume” extends to the base of the lithosphere and has very probably been mobilized through compressional processes that forced the lithosphere to sink into the asthenosphere. The above model is supported by: high heat flow in the Aegean region; low velocity of the compressional waves of 7.7 km/sec for the upper mantle; lower density than normal extending to the base of the lithosphere; teleseismic P-wave travel-time residuals of the order of +2 sec for seismic events recorded at the Greek seismic stations; volcanics in the Aegean area with a chemical composition which can be explained by assuming an assimilation of oceanic crust by the upper mantle; deep seismicity (200 km) which has been interpreted by various authors as a Benioff zone.

Transect across the West Antarctic rift system in the Ross Sea, Antarctica

Abstract In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-rift grabens), the Victoria Land Basin, the Central Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic rift system, with greatest extension beneath the early-rift grabens. The large amount of crustal stretching below the major rift basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the rifting, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with relatively small extension, concentrated in the western half of the Ross Sea, during Cenozoic time.