Giant slope scars and mass transport deposits across the Rhodes Basin, eastern Mediterranean: Depositional and tectonic processes
Introduction
Regionally extensive mass transport deposits have been commonly observed in many marine environments. These include landslides that develop along the flanks of submarine volcanoes, such as those seen in Hawaii (Moore et al., 1989, Moore et al., 1994) and debris flow deposits and turbidites that develop across submarine fans (e.g., Amazon Fan, Piper et al., 1997; Maslin et al., 1998, Laurentian Fan, Piper et al., 1999, Piper and Normark, 2009; Var Fan, Savoy et al., 1993, Migeon et al., 2006) to mention a few. However, such deposits are most ubiquitous along continental slopes (Canals et al., 2004; Gauer et al., 2005; Posamentier and Martinsen, 2011; Somoza et al., 2019). Although also present in passive continental margins, mass transport deposits are more commonly observed across continental slopes of active convergent margins, such as the eastern Mediterranean (Ceramicola et al., 2014; Dennielou et al., 2019). Several factors are required to generate sediment failure and the subsequent mass transport in the marine environment, including high rates of tectonic activity (uplift/subsidence) with frequent large earthquakes, high sedimentation rates leading to increased fluid pressures and the occurrence of gas hydrates (McAdoo et al., 2000; Collot et al., 2001).
Submarine slumps and landslides, seafloor failures, mass flow deposits and mass transport deposit are all nonspecific terms that refer to material that is geologically rapidly remobilized from, and redeposited on the seafloor. Herein, the term mass transport deposit is used for a seismic package that is believed to have been deposited through a single event, following the recommendation of Pickering and Hiscott (2016). All the above terms imply failure (often across the upper slope regions) and the subsequent gravitational deformation in the form of downslope mass movements and flows (Nardin et al., 1979; Coleman and Prior, 1988; Hampton et al., 1996; Mulder and Cochonat, 1996; Canals et al., 2004). Seafloor processes leading to mass transport deposits include (a) slides and slumps, where failure is associated with discrete sliding surfaces, often with little internal deformation (e.g., Masson et al., 2002) and (b) mass flows, including grain, mud and debris flows, where there is variably large internal deformation resulting from laminar fluid-like motion (Prior et al., 1984; Dasgupta, 2003). Turbidity currents can also originate from submarine mud and debris flows, when the in-mixing of the surrounding water dilutes the flow, leading to sudden liquefaction. During this process the pore-water pressure increases and shear strength dramatically decreases (nearing zero) and the flow is transformed into a cohesionless mass that travels downslope. Unlike the mud and debris flows where the flow is supported by grain-to-grain interactions, in the turbidity currents the flow is supported by upward-moving pore pressure and turbulence. Various types of mass transport deposits can develop within the same environment such as the submarine avalanche/slump complexes (e.g., Masson et al., 2002), debris avalanches and debris flows (e.g., Collot et al., 2001), slumps and debris flows (e.g., Piper et al., 1999; Jenner et al., 2007) and mass transport complexes stacked on top of one another (e.g., Moscardelli et al., 2006). Furthermore, several transport and depositional processes can also occur simultaneously across a margin (Nardin et al., 1979) or sequentially where a particular process can transform itself into another, such as debris flows evolving into turbidity currents (Mohrig and Marr, 2003; Felix and Peakall, 2006), slides evolving into debris flows (Prior et al., 1984; Masson et al., 1998), and slumps evolving into debris avalanches (Lamarche et al., 2008). In the marine environment mass transport deposits span several orders of magnitude in area (<100–1010 m2), and volume (103–1012 m3; Lamarche et al., 2008; Pickering and Hiscott, 2016). Runout distances of such transports may be several hundred kilometers (e.g., Storegga slide, Kvalstad et al., 2005).
In this paper we present volumetrics data on slope failures and mass transport deposits across the Rhodes Basin and environs (Fig. 1, Fig. 2). We calculate the total mass of the uppermost Messinian–Quaternary sediments contained within the Rhodes, Finike and Anaximander basins and compare this mass with the mass of sediments delivered into the northwestern sector of the eastern Mediterranean by the present-day small rivers, assuming that the present-day discharges of these rivers can be used as a proxy for the uppermost Messinian–Quaternary discharges. We further calculate the masses of the sediments evacuated from the northern and northeastern continental margins of the Rhodes Basin during the uppermost Messinian–Lower Pliocene and compare and contrast these masses with the mass of the mass transport deposits mapped across the Rhodes Basin. In doing so, we aim to provide a set of summary concluding criteria that may serve as a model for the evolution of basins across tectonically active regions of the Mediterranean and elsewhere around the world.
Section snippets
Seismic reflection profiles
The high-resolution seismic reflection profiles used in this paper (~5500 line-km) were collected in 2001, 2007 and 2010 using RV Koca Piri Reis. Details of the acquisition parameters are fully described in Aksu et al. (2019). Seismic stratigraphic thicknesses are expressed in milliseconds two-way time (or ms twt). All conversion from ms twt to metre is made assuming 1500 m s−1 interval velocities.
Stream loads and yields
In this paper, rivers are defined as those water courses with annual average discharge ≥10 m3 s−1,
Regional setting
Rhodes Basin is a prominent near-circular deep depression in the northern sector of the eastern Mediterranean, situated immediately east of the Island of Rhodes and southwest of mainland Turkey (Fig. 1). The western extensions of the Anaximander Mountain (sensu stricto) and the Sırrı Erinç Plateau collectively define the southern margin of the Rhodes Basin. The southwestern sector of the basin is marked by a narrow NE–SW trending depression representing the Strabo Trench (Fig. 1). The Rhodes
Morphology of the northern continental margin of the Rhodes and Finike basins
The northern continental margins of the Rhodes and Finike basins exhibit two distinct morphologies: regions characterized by very smooth seafloor and very rugged seafloor (Fig. 2). For example, region immediately northeast of the northern tip of the Island of Rhodes exhibits very smooth seafloor morphology: here 4–6 coast-parallel bathymetric steps are visible on the high-resolution multibeam images (Fig. 2, Fig. 3). Similarly, the region northeast Rhode continental margin also exhibits smooth
Slope angles across the Anaximander Mountains, Sırrı Erinç Plateau and Rhodes and Finike basins
The slope gradients across the study area range from near 0° in deep basinal settings of the Rhodes and Finike basins, to >50° locally across the region exhibiting jagged seafloor morphologies (Fig. 4). Two distinct trends are discernible: (a) high slope gradients mark the fringes of many prominent structural elements, such as seen across the Sırrı Erinç Plateau, which also include the Anaximander Mountain (sensu stricto), the northwestern sectors of the Anaximenes and Anaxagoras mountains and
Seismic stratigraphy and chronology
There is a relatively dense grid of high-resolution seismic reflection profiles (collected, processed and interpreted by the authors) across the Rhodes and Finike basins and the Anaximander Mountains (sensu lato), including the Sırrı Erinç Plateau (Fig. 6). The seismic chronostratigraphic framework of these data is previously published in Aksu et al., 2009, Aksu et al., 2018, Aksu et al., 2019. These previous studies identified four seismic stratigraphic package: Units 1–3, separated by two
Results
The results are presented in three sections: (a) comparison between the volumes and masses of the uppermost Messinian–Quaternary sediments (i.e., Unit 1) between the Finike, Rhodes and Anaximander basins; (b) description of the morphology of the northern and northeastern continental margin showing widespread scabland-like regions where older basement successions are exposed on the seafloor, and (c) comparison between the volume and mass of the mass transport deposits across the Rhodes Basin and
Discussion
Previous studies showed that the Island of Rhodes, the Rhodes and Finike basins and the present-day Anaximander Mountain (sensu stricto) experienced dramatic tectonic activities during the Pliocene–Quaternary (e.g., van Hinsbergen et al., 2007; Hall et al., 2009, Hall et al., 2014; Aksu et al., 2019). For example, the Island of Rhodes experienced 9 ± 6° counterclockwise vertical axis rotation during the middle Pliocene (~3.8–3.6 Ma), followed by SE-tilting and 500–600 m subsidence of the
Conclusions
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Examination of multibeam mosaic maps of the seafloor across the southwestern Turkish continental margin revealed the presence of two prominent regions: one characterized by very sugged morphology (refereed to as Scars 1 and 2), and another characterized by very smooth seafloor morphology. The upper slope regions across the rugged seafloor include several concave, interconnected steep seafloor escarpments marked by semi-circular indentations that link with one another by cusp-like features
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank the officers and crew of the RV Koca Piri Reis of the Institute of Marine Sciences and Technology, Dokuz Eylül University for their assistance in data acquisition, in particularly the former Captains Mehmet Özsaygılı and Kemal Dursun and the former Chief Engineers Bilal Nuriler and Ömer Çubuk for their invaluable assistance during the many years of geophysical operations. We further thank Dr. Doğan Yaşar of the Institute of Marine Sciences and Technology, Dokuz Eylül University for his
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