Cretan Basin Research Articles

The Cretan Basin (South Aegean Sea, NE Mediterranean) in the Early Pliocene: a paleoceanographic reconstruction

The Aegean Sea (NE Mediterranean) represents a key area for climatic and oceanographic future projections, therefore it is crucial to define its environmental evolution during the Pliocene paleoclimatic analogue. The DSDP Site 378, located in the South Aegean Cretan Basin, provides an exceptional opportunity to investigate in detail the paleoceanographic conditions of the Early Pliocene and to examine the basin's paleoenvironmental evolution, through the study of calcareous nannofossil accumulation rates and hydroclimate proxies. The sequence has been dated in between ∼5.2–3.9 Ma, with the age of the first Zanclean marine sediments overlying the Messinian Primary Lower Gypsum demonstrating that the Cretan Basin was partially desiccated for at least ∼400 kyrs, as a result of being a shallow epicontinental area in the Late Miocene. A relatively sudden change to deep marine environment took place in the Early Pliocene linked to the accelerated rate of trench extension in the Cretan Basin. The multi-proxy study revealed intervals reflecting warm surface waters and low-oxygen conditions on the seafloor, associated with the orbitally driven presence of Early Pliocene rhythmic sapropelic layers. Between ∼5.2 and 4.6 Ma, the sapropelic layers were characterized by elevated abundance of Reticulofenestra spp. and Florisphaera profunda, depicting increased productivity throughout the photic zone. Both enhanced riverine inputs and weak presence of Levantine Intermediate Water (LIW) characterized this time interval. The alterations in the calcareous nannofossil patterns with increased Helicosphaera spp. and Umbilicosphaera jafari abundances, implying freshening and enhanced mid-photic productivity after ∼4.6 Ma, suggest strengthening of LIW intrusion combined with the full restoration of North-South Aegean marine gateways that led to the establishment of a hydrographic system similar to the modern Aegean Sea.

Early Pliocene calcareous nannofossils contribute to the paleoceanographic reconstruction of the Cretan Basin (southern Aegean Sea, NE Mediterranean).

Early Pliocene calcareous nannofossils contribute to the paleoceanographic reconstruction of the Cretan Basin (southern Aegean Sea, NE Mediterranean).

Oceanographic Signatures and Pressure Monitoring of Seafloor Vertical Deformation in Near-coastal, Shallow Water Areas: A Case Study from Santorini Caldera

ABSTRACTBottom pressure, tilt, and seawater physical properties were monitored for a year using two instruments within the immerged Santorini caldera (Greece). Piggybacked on the CALDERA2012 cruise, this geodetic experiment was designed to monitor evolution of the 2011–2012 Santorini unrest. Conducted during a quiescent period, it allowed us to study oceanographic and atmospheric signal in our data series. We observe periodic oceanographic signals associated with tides and seiches that are likely linked to both the caldera and Cretan Basin geometries. In winter, the caldera witnesses sudden cooling events that tilt an instrument towards the Southeast, indicating cold water influx likely originating from a passage into the caldera between Thirasia island and the northern end of Thera island to the northwest. We did not obtain evidence of long-term vertical seafloor deformation from the pressure signal, although it may be masked by instrumental drift. However, tilt data suggest a local seafloor tilt event ∼1/year after the end of the unrest period, which could be consistent with inflation under or near Nea Kameni. Seafloor geodetic data recorded at the bottom of the Santorini caldera illustrate that the oceanographic signature is an important part of the signal, which needs to be considered for monitoring volcanic or geological seafloor deformation in shallow water and/or nearshore areas.

Hydrodynamic features of the South Aegean Sea as derived from Argo T/S and dissolved oxygen profiles in the area

In this work, the hydrodynamic picture of the South Aegean Sea is examined through an analysis of recent measurements in its sub-basins, the Myrtoan and Cretan Sea. Both sub-basins play an important role in the water circulation, exchange, and formation processes that affect the dynamics of the whole Eastern Mediterranean. For the first time, Bio-Argo floats were deployed in the area under the Greek Argo Research Infrastructure coordination. The acquired profiles cover an almost 2-year period (November 2013–July 2015) and are compared with previous Argo profiles and the re-processed time-series data recorded from the E1-M3A POSEIDON observatory operating in the area since 2007. The spatio-temporal distribution of the physical and chemical properties in each sub-basin is examined. Dense water formation events are revealed in the northern part (Myrtoan), while the wider area can be characterized as pre-conditioned. In the Cretan basin, a strong inter-annual variability of the salinity field at intermediate and deep layers is observed that is associated with water exchange from its open boundaries. Furthermore, comparison of the dissolved oxygen (DO) distribution with physical water properties within both the mixed layer, and at greater depths, indicated that relatively high but still under-saturated DO values are more likely to be associated with convection events. Finally, an updated picture of the physical properties and the DO distribution is presented based on the last 5 years of measurements and the recent introduction of Bio-Argo floats with DO sensors in the area.

Recurrent intrusions of transitional waters of Eastern Mediterranean origin in the Cretan Sea as a tracer of Aegean Sea dense water formation events

Available temperature and salinity data in the Cretan Sea from 1955 up to 2014 as well as literature sources were revisited in order to trace the appearance of low salinity, temperature, oxygen and nutrient-rich waters inside the basin at depths below the intermediate layer. First appearing as far back as 1961 in literature, these waters were found originating in the layers that separate intermediate and deep waters of the Eastern Mediterranean Sea (EMed) and were named Transitional Mediterranean Water (TMW) in the 1990s. Data analysis showed that the appearance of TMW in the Cretan Sea is a recurrent phenomenon connected to water mass exchanges between the Aegean Sea and the EMed. In particular, the inflow of TMW in the Cretan basin acts as compensation for the outflow of equally dense or denser masses from the Aegean. This export is a result of dense water formation (DWF) events taking place inside the Aegean Sea triggering TMW compensatory inflow into the Cretan Sea through the Cretan Straits. In this context, TMW intrusions in the Cretan basin can be used as a tracer of DWF in the Aegean Sea while the depth of the intrusion can provide valuable information about the intensity of the DWF event. The importance of TMW intrusions is not solely restricted to the tracing and evaluation of DWF events but could additionally expand to the impact on local ecological processes as TMW is a nutrient carrier for the oligotrophic Cretan Sea.

The January 2012 earthquake sequence in the Cretan Basin, south of the Hellenic Volcanic Arc: Focal mechanisms, rupture directivity and slip models

The January 2012 moderate-size (Mw5.3) sequence in Cretan Basin is studied, in an attempt to shed light to the deformation pattern along the central part of the southern Aegean Sea, south of the Hellenic Volcanic Arc. The two strongest events of the earthquake sequence, occurred on January 26 (UTC 04:24 Mw5.0) and on January 27 (UTC 01:33; Mw5.3), with epicenters lying 25km and 50km SW of the Christiani Isles and Santorini volcanic fields, respectively. Moment tensor inversions revealed the operation of pure strike–slip motions, along N–S and E–W trending vertical planes. The deviatoric moment tensor solutions, in a number of cases, involved more than 30% non-double couple component. The seismicity was confined in a vertical column of small (~5km) horizontal extent, whereas the best constrained depths were in the range of 5 to 14km. The good station coverage and the striking similarity of the waveforms permitted directivity effects to be resolved, from the shape and amplitude of the apparent Source Time Functions (STFs), obtained through an empirical Green's function approach. The rupture directivity towards south, resolved the N–S trending plane as the fault plane for both of the two strongest events. This sequence provided evidence that the WNW–ESE trench-parallel extension accommodated within the overriding Aegean plate, which so far was well documented in the western and eastern parts, it is also operating in the central part of the southern Aegean Sea. The slip models for the two strongest events were simple, single patched and the calculated static stress drop was of the order of 35 bars, in agreement with the average value in the back-arc Aegean Sea region.

Thrust faults and extensional detachment faults in Cretan tectono-stratigraphy: Implications for Middle Miocene extension

The revised tectono-stratigraphy of Crete and especially of the “Phyllites–Quartzites” complex demonstrated the distinction of the probable Paleozoic low-medium grade metamorphic rocks of the Arna unit from the underlying Permo–Triassic phyllites and associated carbonate sediments (Trypali facies) of Western Crete unit as well as the overlying Permo–Triassic phyllites and associated sediments of the Tyros/Ravdoucha Beds at the base of the Tripolis unit. The pre-existing mixture of the above tectono-stratigaphic units in a single complex created a number of misinterpretations as far as stratigraphy, metamorphism and interpretation of low angle faults as thrusts or detachments. Especially in cases where the inferred tectonic contact concerns the transition between the Tyros Beds and the base of the Tripolis carbonate platform there is no structural omission and therefore the contact represents a minor disharmonic sliding surface and not a detachment. Based on the revised tectono-stratigraphic analysis the determination of the structural omission for each tectonic contact was possible and several detachments were described for the first time. Footwall rocks of the detachments comprised several tectonic units usually from the lower nappes and hanging wall rocks comprised several tectonic units usually from the upper nappes. The detachment may separate not only metamorphosed units in the footwall (Mani, Western Crete, Arna) from non metamorphosed units in the hanging wall (Tripolis, Pindos and higher nappes) but also all other possible combinations from the Cretan nappe pile. Extension in Crete started in the Middle–Late Miocene with the formation of extensional detachment faults. The reported extensional structures of Oligocene to Early Miocene age do not correspond to crustal extension of Crete but to localized shear zones related to nappe stacking and the exhumation of metamorphic rocks. Extensional detachments in Crete form a tectonic horst through two oppositely dipping E–W-trending zones; one dipping north, related to the opening of the Cretan basin, and the other dipping south, related to the formation of the Messara supra-detachment basin. The deformation history of units within Crete can be summarized as: (i) compressional deformation producing arc-parallel east–west-trending south-directed thrust faults in Oligocene to Early Miocene time; (ii) extensional deformation along arc-parallel, east–west-trending detachment faults in Middle Miocene time, with hanging wall motion to the north and south; and (iii) Late Miocene–Quaternary transtensional deformation along high-angle normal and oblique normal faults that disrupt the older arc-parallel structures.

The anatomy and provenance of thick volcaniclastic flows in the Cretan Basin, South Aegean Sea

Santorini volcano has been the largest source of volcaniclastic sediment in the Eastern Mediterranean during the late Quaternary. A dozen cores from the Cretan Basin, south of Santorini, have sampled two megabeds that consist of gravity emplaced volcaniclastic sequences. The uppermost megabed U consists of a succession of five (U 5–U 1) base cut out turbiditic units. Lower megabed A is a single turbiditic event. Only the uppermost U 2 and U 1 turbidites are separated from the underlying beds by hemipelagic marls. The texture and composition of the U and A megabeds closely match the texture and composition of the fine, vitric ash of the “Minoan” deposits on Santorini islands, dating from about 3500 yr BP. These megabeds are therefore attributed to rapid accumulation of separate gravity flows fed by the “Minoan” eruption, except for the upper U 2 and U 1 turbidites deposited from subsequent gravity flows transporting eroded volcaniclastic sediments. With the exception of the margin south of Santorini, dozens of cores retrieved around the margins of the Cretan Basin have a continuous late Quaternary succession that shows no evidence for massive sediment remobilization into the deeper basin, including the passage of the “Minoan” tsunami. Extensive high-resolution 3.5 kHz records revealed the acoustic character, architecture and distribution of the U and A megabeds and four underlying late Quaternary volcanogenic megabeds in the Cretan Basin. The acoustic facies of megabeds are typical of megaturbidites and consist of an upper, transparent, lower velocity layer that corresponds to the fine-grained upper turbiditic silt and clay section and a lower, strongly reflective higher velocity section that corresponds to the lowest, coarser-grained base of the turbidite that is developed over a sharp erosional surface. Penetration of the high-resolution records reveals the existence of at least six megabeds. Correlation with core lithology and the physical properties of the various lithofacies, including down-core velocity profiles, has allowed us to determine the thickness and volumes of the upper four megabeds which are: U ≤ 9 m thick, volume 3.7 km 3; A ≤ 25 m thick, volume 12.2 km 3; B ≤ 22 m thick, volume 10.3 km 3; C ≤ 15 m thick, volume 8 km 3. These thick megabeds are the uppermost products of repeated explosive eruption of Santorini in the late Quaternary. Calculated sedimentation rates from and after the “Minoan” eruption are 9.4 m/1000 yr that rise to over 15.7 m/1000 yr if megabed B was also deposited during this eruption.

Aegean Sea Water Masses during the Early Stages of the Eastern Mediterranean Climatic Transient (1988–90)

Abstract The Aegean water masses and circulation structure are studied via two large-scale surveys performed during the late winters of 1988 and 1990 by the R/V Yakov Gakkel of the former Soviet Union. The analysis of these data sheds light on the mechanisms of water mass formation in the Aegean Sea that triggered the outflow of Cretan Deep Water (CDW) from the Cretan Sea into the abyssal basins of the eastern Mediterranean Sea (the so-called Eastern Mediterranean Transient). It is found that the central Aegean Basin is the site of the formation of Aegean Intermediate Water, which slides southward and, depending on their density, renews either the intermediate or the deep water of the Cretan Sea. During the winter of 1988, the Cretan Sea waters were renewed mainly at intermediate levels, while during the winter of 1990 it was mainly the volume of CDW that increased. This Aegean water mass redistribution and formation process in 1990 differed from that in 1988 in two major aspects: (i) during the winter of 1990 the position of the front between the Black Sea Water and the Levantine Surface Water was displaced farther north than during the winter of 1988 and (ii) heavier waters were formed in 1990 as a result of enhanced lateral advection of salty Levantine Surface Water that enriched the intermediate waters with salt. In 1990 the 29.2 isopycnal rose to the surface of the central basin and a large volume of CDW filled the Cretan Basin. It is found that, already in 1988, the 29.2 isopycnal surface, which we assume is the lowest density of the CDW, was shallower than the Kassos Strait sill and thus CDW egressed into the Eastern Mediterranean.

Short-term climate changes in the southern Aegean Sea over the last 48,000 years

High-resolution palaeoenvironmental changes, corresponding to a mean time interval of 450 years covering the last 48,000 years, were examined in a core from the Cretan Basin in the southern Aegean Sea. The intensity and duration of the climatic and oceanographic events were determined by examining the compositional changes in the planktonic foraminifera and pollen assemblages, along with the δ 18O signal of Globigerinoides ruber. A reconstruction of sea-surface temperatures was attempted using the Modern Analogue Technique (MAT). In total, 10 stadials and 6 interstadials occurred over the last 48,000 years. These fluctuations in climatic conditions coincide with fluctuations documented in the western and central Mediterranean and seem to be associated with Dansgaard–Oeschger events. Some of these climatic fluctuations are correlated with changes in the vegetation in the surrounding land. Between 48 and 10 cal kyr BP the most pronounced stadials occurred at 41 cal kyr BP (C69-ST10) and at 13 cal kyr BP (C69-ST4). These events are characterized by: (i) high positive δ 18O values of Globigerinoides ruber, (ii) drops in SST and (iii) increases in aridity. These events may be correlated with the Heinrich H4 event and the Younger Dryas event, respectively. Two other stadials at 23 cal kyr BP (C69-ST6) and at 16 cal kyr BP (C69-ST5) which are characterized by increases in the abundance of the cold plaktonic foraminifera species and increases in aridity may be correlated with the H2 and H1 events, respectively. The dominant planktonic foraminiferal species during the stadials witch are correlated with the Heinrich events were Turborotalita quinqueloba and Globorotalia scitula. The most pronounced interstadials occurred between 39.5 and 38.5 cal kyr BP (C69-IST6) and between 25 and 24 cal kyr BP (C69-IST3) and are characterized by depletion in δ 18O values, increases in SST and increases in humidity. The former event coincides with the formation of the sapropelitic layer S2. In the Holocene the most pronounced stadial occurred between 8 and 6.5 cal kyr BP (C69-ST2), during the interruption of S1 and is characterized by a reduction in SST and an increase in aridity. The most pronounced interstadials of Holocene occurred during the formation of S1a and S1b between 9 and 8 cal kyr BP (C69-IST1) and between 6.5 and 5.5 cal kyr BP (C69-IST2), respectively. These events are characterized by depletion in δ 18O values, increased SST and an increase in humidity as is indicated by the expansion of temperate evergreen and Mediterranean taxa in the pollen record.

Late Holocene multisource material input into the Aegean Sea: depositional and post-depositional processes

The upper ~50 cm sediments in the deep-sea Aegean Sea were generally deposited through hemipelagic rain as indicated by paucity of terrigenous sand-sized particles, abundance of clay-sized materials and lack of turbiditic beds. However, quartz sand-sized grains, black debris, higher C org content and C/N ratio attest to a significant river-borne supply in the North Aegean Trough. Recurrent gravity-induced transfers from the shelf break are evidenced through various markers of coastal deposition dating back the last low-stand ( Characea oogons, Elphidium crispum, Ammonia beccarii, glauconitic grains, evolved organic matter). Especially, this deposition induced significant rise of the C/N ratio of successive layers. The top of ash layers of the Minoan eruption was reached in several sample cores giving opportunity for an estimation of sedimentation rate. The rate values (7–15 cm per kyear) in the southern basin are not significantly lower than those (4–23 cm per kyear) in the northern basin in spite of a lower terrigenous supply. This is due to the abundance of cemented Mg-calcite clasts and nodules throughout the cores of the Cretan Basin and North Skiros Basin rising the carbonate content up to 60%. It is suggested that this high carbonate concentration participates in the relative preservation of the most labile components of the organic matter in a large southern part of the basin characterised by low C/N ratio (5.5–8). Around 2000–1200 year BP, thin darks layers were deposited and are not yet completely oxidised. They would attest to a very beginning stage of sapropelic sediment formation during times of stagnant bottom water probably related to the enhanced Black Sea water circulation.

Quaternary neotectonics of the South Aegean arc

The sedimentary and tectonic Quaternary evolution of the South Aegean arc has been interpreted from 8000 line-km of sparker seismic reflection profiles. The older parts of basins formed by E–W-trending faults and accumulated hemipelagic sediment. Younger active faulting trends NNW in the western part of the arc and ENE in the eastern arc. The ENE-trending faults record sinistral strike-slip motion broadly parallel to the convergence direction of the Aegean–Anatolian and African plates. The north–south faults in the western arc are similar to those on land in the south Peloponnese. Chronology is estimated from sedimentation rates in cores and from stacked progradational units on basin margins controlled by eustatic changes in sea level. The younger fault pattern developed at 0.6–0.8 Ma in the western and central arc, but progressively younger in the southeastern part of the arc. The older basin fill with E–W faults is of early Quaternary and Pliocene age. An older regional unconformity corresponds to an early Pleistocene unconformity in the Cretan basin. Particularly in the area east of Santorini, faulting has resulted in startling changes in palaeogeography, such that some areas were late Pliocene basins, positive features during the Pleistocene and then subsided 600 m in the last 0.2 Ma. Such rapid basin inversion resulted in uplift of stratified hemipelagic muds, which were deformed by creep or failure, so that debris flow deposits accumulated in valleys and ponded basins. These fundamental tectonic changes in the past two million years indicate that much of the deformation resulting from the interaction of the Eurasian, African and Anatolian–Aegean plates is taken up at the southern margin of the Aegean microplate, probably because subduction of the African plate has slowed as a result of collision of continental crust. Regional fault patterns are a consequence of spatially varying effects of subduction roll-back.

Eastern Mediterranean sapropels: chemical structure, deposition and relation to oil-shales

Ten sapropels, deposited in three different basins of the eastern Mediterranean since the Miocene and selected from cores of the Deep Sea Drilling Programme have been characterised by elemental analysis; fluorescence, infra-red and NMR spectrometry, by pyrolysis-gas chromatography/mass spectrometry and by catalytic hydrogenation at moderately high pressure. The sapropels are Types I–II kerogens, which have been oxidised, probably by a front experienced, since their deposition. Only one, from the Cretan basin, contained structures from lignin. The others, typical of a marine deposition, possessed aromaticities of about 0.2. Their detailed organic structures are described.

Subaqueous bedforms on the Cyclades Plateau (NE Mediterranean) — evidence of Cretan Deep Water Formation?

Sandy bedforms are surveyed on the northern–northeastern Cyclades Plateau using side-scan sonar and 3.5 kHz continuous subbottom profiling system. The bedforms appear in water depths of 80–130 m, as; (a) dunes (3-D megaripples; wavelength 10–35 m and height 1–2 m), composed of moderately well-sorted coarse sand; (b) large to very large sand waves (wavelength 50–300 m and height 1.5–6 m) developed usually in fine-medium sand; (c) 2-D megaripples (wavelength 3–5 m and height 0.2–0.4 m); (d) narrow sand ribbons (thin erosional lineations); and (e) elongate sand patches, together with those of different configurations. The presence of these bedforms implies strong near-bed flow, of the order of 40–100 cm/s; and, locally, in the case of sand ribbons, up to 200 cm/s. Short-period (15 day) deployments of near-bed current meters did not observe such values; maximum and mean velocities were 10–15 and 6–7 cm/s, respectively. Indirect evidence, provided by the local fishermen, is indicative of strong currents. Dense water mass formation, during cold winters, is considered to be responsible for the development of these bedforms. During seasonal cooling, such waters sink rapidly and flow towards the south, flooding the deeper part of Cretan Basin (Cretan Deep Water (CDW) formation); they overflow the Cretan Straits and are dispersed into the Eastern Mediterranean. Sites for this formation of CDW are considered to be the Cyclades Plateau and possibly the North and the Central Aegean Sea.

96/02215 13C solid-state n.m.r. spectra of Shanxl coals

Ten sapropels, deposited in three different basins of the eastern Mediterranean since the Miocene and selected from cores of the Deep Sea Drilling Programme have been characterised by elemental analysis; fluorescence, infra-red and NMR spectrometry, by pyrolysis-gas chromatography/mass spectrometry and by catalytic hydrogenation at moderately high pressure. The sapropels are Types I–II kerogens, which have been oxidised, probably by a front experienced, since their deposition. Only one, from the Cretan basin, contained structures from lignin. The others, typical of a marine deposition, possessed aromaticities of about 0.2. Their detailed organic structures are described.

On late miocene to recent vertical motions in the Cretan segment of the Hellenic arc

High-resolution foraminiferal and calcareous nannoplankton biostratigraphy and reliable paleobathymetry estimates based on plankton-benthos ratios in foraminifera make it possible to reconstruct in detail Late Neogene vertical motions along the Central Cretan segment of the Hellenic arc. These motions are considered to express the surficial effect of the roll-back process of the Hellenic subduction zone, which started about 12 Ma ago. In contrast to earlier views there is no sustained uplift since the late Middle Miocene. Successive paleotopographic profiles for central Crete show a predominance of subsidence, coupled with an increase of differential reliefs, from the latest Serravallian until the Messinian. Subsidence was most pronounced between the Tortonian-Messinian boundary interval and the early part of the Early Pliocene, locally up to a magnitude of more than 1000 m. A two-phased uplift history, coupled with tilting to the north or northeast, can be inferred from the late Early Pliocene-Recent record separated by a short, early Late Pliocene episode of subsidence. Rates of uplift were highest in the Early Pliocene up to about 125 cm/ka. The Central Cretan Late Neogene to Recent paleotopographic profiles are compared with those reconstructed from seismic interpretations and drilling data from the Cretan Sea. The combined results show that paleotopographic configurations between the Cyclades and Crete were fairly similar to those on central Crete until Messinian time. Foundering of the Cretan Basin started in the course of the Early Pliocene, but subsidence rates were less than the contemporaneous uplift rates of Crete. The timing and magnitude of the vertical motions along the Cretan Sea-Central Cretan transect put detailed geological constraints on tectonophysical modelling of the surficial effects of the roll-back process. These vertical motions are discussed in view of the model of southward translation of a large supracrustal slab, resulting in the origin of the Cretan Sea and in thrusting and uplift in the frontal parts of the slab, where it thrusted over the northern limb of the subducted Ionian Plate.