Elsevier

Marine Geology

Volume 263, Issues 1–4, 15 July 2009, Pages 97-107
Marine Geology

New evidence for massive gravitational mass-transport deposits in the southern Cretan Sea, eastern Mediterranean

https://doi.org/10.1016/j.margeo.2009.04.002Get rights and content

Abstract

Newly acquired bathymetric and seismic reflection data have revealed mass-transport deposits (MTDs) on the northeastern Cretan margin in the active Hellenic subduction zone. These include a stack of two submarine landslides within the Malia Basin with a total volume of approximately 4.6 km3 covering an area of about 135 km2. These two MTDs have different geometry, internal deformations and transport structures. The older and stratigraphic lower MTD is interpreted as a debrite that fills a large part of the Malia Basin, while the second, younger MTD, with an age of at least 12.6 cal. ka B.P., indicate a thick, lens-shaped, partially translational landslide. This MTD comprises multiple slide masses with internal structure varying from highly deformed to nearly undeformed. The reconstructed source area of the older MTD is located in the westernmost Malia Basin. The source area of the younger MTD is identified in multiple headwalls at the slope–basin-transition in 450 m water depth. Numerous faults with an orientation almost parallel to the southwest–northeast-trending basin axis occur along the northern and southern boundaries of the Malia Basin and have caused a partial steepening of the slope–basin-transition. The possible triggers for slope failure and mass-wasting include (i) seismicity and (ii) movement of the uplifting island of Crete from neotectonics of the Hellenic subduction zone, and (iii) slip of clay-mineral-rich or ash-bearing layers during fluid involvement.

Introduction

Submarine landslides are commonly observed at continental margins worldwide. Many studies (e.g. Locat and Lee, 2002, Sultan et al., 2004, Canals et al., 2004, Masson et al., 2006) have shown that earthquake activity and oversteepening of slopes, as well as wave activity, sealevel changes, and rapidly increasing sediment load are some of the most common trigger mechanisms for slope failure. In addition, gas hydrate dissociation may play an important role for slope destabilization (e.g. Gunn et al., 2002, Mienert et al., 2003, Vendeville and Gaullier, 2003). Pore pressure increase is a common cause of low effective stresses, and may hence be another potential generator of slope instability (Bromhead, 1992, Sultan et al., 2004), for instance by liquefaction of ash-layers (Carey, 1997, Kutterolf et al., 2008). Submarine landslides are commonly initiated by transient pore-pressure conditions associated with the variety of above-mentioned geological processes (see summary in Hampton et al. (1996), and Locat and Lee (2002)).

One region that concentrates several of these geological processes is the tectonical active Hellenic subduction zone in the eastern Mediterranean with the Island of Crete in its centre (Fig. 1). Various data sets (e.g. Mascle and Martin, 1990, Fassoulas, 2000, Manakou and Tsapanos, 2000) indicate earthquake activity and active faults in the vicinity of Crete. Thereby, the accretionary prism and its continental crust backstop south of Crete are tectonically more active (Chaumillon and Mascle, 1996, Polonia et al., 2002, Kopf et al., 2003, Chamot-Rooke et al., 2005), indicating a generally higher potential for slope destabilization, demonstrated by large-scaled landslides identified along the southern margin of Crete, e.g. the Lithinon Slide (Huson and Fortuin, 1985). Along the northern Cretan slope submarine landslides have rarely been studied. This area is characterized by high sedimentation rates of 8–22 cm ka 1 (Chronis et al., 2000b) and by fine-grained mixed hemipelagic and terrigenous sediments with several interbedded sapropels and volcanic ashes in shallow and mid-slope regions (Chronis et al., 2000a, Kopf et al., 2007). Consequently, potentially unstable slope areas and large landslide events are expected also for the northern Cretan slope. Chronis et al. (2000b) already provided evidence for gravitational mass-transport on the northern Cretan mid-slope. However, detailed studies are not available at present.

We collected a dense grid of bathymetric data and multichannel reflection seismic profiles from the southern Cretan Sea sub-basins and along the northern Cretan slope in water depths of 150 m to 2800 m during R/V-Poseidon Cruise P-336 “CRESTS” (Cretan Sea Tectonics and Sedimentology; Fig. 1). Main objectives of the CRESTS cruise were to study slope instability and related geohazards at the northern Cretan Margin, and to acquire site survey for potential future scientific drilling (see also Kopf et al., 2006, Kopf et al., 2007). In addition to the acoustic data, 30 gravity cores of 1 m to 5.7 m length were recovered along transects on seismic lines.

In this study, we (i) identify and characterize MTDs in an area off shore the Cretan town of Malia, (ii) reconstruct the sediment source areas, and (iii) propose a conceptual model for sediment transport mechanisms and possible triggers.

Section snippets

Geological setting along the Hellenic subduction zone

The east–west-trending, 1500 km long and 200 km wide Mediterranean Ridge is a major structure of the convergent margin between the African and Eurasian plate (Fig. 1). It is bordered to the south by the subduction trench and to the north by the island of Crete, which represents an exhumed forearc-high acting as a backstop for the accreted strata (e.g. Le Pichon et al., 2002, Polonia et al., 2002, Kopf et al., 2003). Continuous uplift of Crete combined with fast sediment accretion along the

Methods

The geophysical data acquisition of Cruise P-336 included a dense grid of swath bathymetric data recorded with an ELAC SEABEAM 1050. This system is operated at 12 kHz with a maximum angular coverage of 150° corresponding to a swath width of up to 7.5 times the water depth. Bathymetric data were processed using MB-System (Caress and Chayes, 2006) and gridded with a resolution of 20 m cell-size using GMT (Generic Mapping Tool; Wessel and Smith, 1998). The bathymetry of the investigated area

Bathymetric data

The area surveyed is occupied by an elongated basin of about 25 km in length and up to 15 km in width with a southwest–northeast-trending axis that smoothly dips to the northwest towards the Kamilonisi Basin (Fig. 2A). Water depth ranges from 170 m to 730 m (Fig. 3A). Owing to its proximity to the city of Malia at the Cretan coast, we name it the Malia Basin (Fig. 2A).

The Malia Basin is bordered to the south by the continental slope of Crete (Fig. 3). To the northwest a morphological high

Seismic units and seafloor textures

Based on structures identified from seismic data in correlation with surface analysis of bathymetry data, seismic Units 1 and 2 are interpreted as mass-transport deposits (MTDs) of two mass-movement events.

Unit 1 is identified as MTD, because the degree of deformation of the reflection pattern is high, which suggests deposition of chaotic masses and sediment amalgamation. The areal distribution of this MTD 1 mostly coincides with the rough Seafloor Texture C, which is consistent with

Conceptual model

The interpretation of the two gravitational mass-movement events in the Malia Basin is based on the deposits' variations in surface roughness, geometry, thickness and degree of internal deformation yielding different transport mechanisms and erosional behaviours. Because there is neither an evidence for background sedimentation nor for erosion between the two MTDs, we assume that their emplacement took place within a short time lapse, whose sedimentary record is thinner than the vertical

Summary and conclusions

The analysis of a dense net of acoustic data in combination with some core data in the Mali Basin north of Crete allows a detailed discussion of mass-wasting in this region for the first time. The main conclusions are:

  • (1)

    We identified two stacked MTDs with a total volume about 4.5 km3 over an area of 135 m2. The two deposits represent the youngest events of repeating mass-wasting activity in the Malia Basin. A younger, massive MTD is identified as an aggregation of slide masses derived from

Acknowledgments

We thank the whole scientific party and the ship crew of P-336 for faithful and detailed recording of data. We thank all involved scientists at the Hellenic Centre for Marine Research, Greece, for their scientific input and discussions as well as support of the P-336 cruise. We want to thank the MARUM for scientific support and all involved scientists and members for fruitful discussions and outcomes. We thank reviewer Angelo Camerlenghi and a second, unknown reviewer for their dedicative work

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