Active tectonic morphology and submarine deformation of the northern Gulf of Eilat/Aqaba from analyses of multibeam data

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A high-resolution marine geophysical study was conducted during October-November 2006 in the northern Gulf of Aqaba/Eilat, providing the first multibeam imaging of the seafloor across the entire gulf head spanning both Israeli and Jordanian
  ORIGINAL Active tectonic morphology and submarine deformationof the northern Gulf of Eilat/Aqaba from analysesof multibeam data Gideon Tibor  &  Tina M. Niemi  &  Zvi Ben-Avraham  &  Abdallah Al-Zoubi  & Ronnie A. Sade  &  John K. Hall  &  Gal Hartman  &  Emad Akawi  & Abdelrahmem Abueladas  &  Rami Al-Ruzouq Received: 23 September 2009 /Accepted: 25 January 2010 # Springer-Verlag 2010 Abstract  A high-resolution marine geophysical study wasconducted during October-November 2006 in the northernGulf of Aqaba/Eilat, providing the first multibeam imaging of the seafloor across the entire gulf head spanning both Israeliand Jordanian territorial waters. Analyses of the seafloor morphology show that the gulf head can be subdivided intothe Eilat and Aqaba subbasins separated by the north-south-trending Ayla high. The Aqaba submarine basin appearsstarved of sediment supply, apparently causing erosion and alandward retreat of the shelf edge. Along the eastern border of this subbasin, the shelf is largely absent and its margin isinfluenced by the Aqaba Fault zone that forms a steep slope partially covered by sedimentary fan deltas from the adjacent ephemeral drainages. The Eilat subbasin, west of the Aylahigh, receives a large amount of sediment derived from theextensive drainage basins of the Arava Valley (Wadi  ’ Arabah)and Yutim River to the north  –  northeast. These sediments andthose entering from canyons on the south-western border of this subbasin are transported to the deep basin by turbiditycurrents and gravity slides, forming the Arava submarine fan.Large detached blocks and collapsed walls of submarinecanyons and the western gulf margin indicate that masswasting may be triggered by seismic activity. Seafloor lineaments defined by slope gradient analyses suggest that the Eilat Canyon and the boundaries of the Ayla high alignalong north- to northwest-striking fault systems  —  the EvronaFault zone to the west and the Ayla Fault zone to the east. Theshelf   –  slope break thatlies along the 100 m isobath in the Eilat subbasin, and shallower (70  –  80 m isobaths) in the Aqabasubbasin, is offset by approx. 150 m along the eastern edge of the Ayla high. This offsetmight bethe resultofhorizontalandvertical movements along what we call the Ayla Fault on theeast side of the structure. Remnants of two marine terraces at 100 m and approx. 150 m water depths line the southwest margin of the gulf. These terraces are truncated by faultingalong their northern end. Fossil coral reefs, which have asimilar morphological appearance to the present-day, basinmargin reefs, crop out along these deeper submarine terracesand along the shelf   –  slope break. One fossil reef is exposed onthe shelf across the Ayla high at about 60  –  63 m water depth but is either covered or eroded in the adjacent subbasins. Theoffshore extension of the Evrona Fault offsets a fossil reef along the shelf and extends south of the canyon to linear fractures on the deep basin floor. Introduction The Gulf of Eilat/Aqaba is the northeast arm of the Red Seathat follows the southern 180 km of the Dead SeaTransform (DST) fault system (Fig. 1a). The gulf formed G. Tibor ( * )Israel Oceanographic and Limnological Research,Haifa 31080, Israele-mail: M. NiemiDepartment of Geosciences, University of Missouri-Kansas City,Kansas, MO 64110, USAZ. Ben-Avraham : G. HartmanDepartment of Geophysics & Planetary Sciences,Tel-Aviv University,Tel-Aviv 69978, IsraelA. Al-Zoubi :  E. Akawi : A. Abueladas :  R. Al-RuzouqSurveying and Geomatics Department,Al-Balqa ’  Applied University,Al Salt 19117, JordanR. A. Sade :  J. K. HallGeological Survey of Israel,Jerusalem 95501, IsraelGeo-Mar Lett DOI 10.1007/s00367-010-0194-y  with the initiation of Red Sea rifting in the Late Oligoceneto Early Miocene (e.g. Ben-Avraham and Garfunkel 1986). Geophysical data indicate that the Gulf of Eilat/Aqabacontains an en echelon arrangement of basins (Fig. 1b)formed between left-stepping, strike-slip faults (Ben-Avraham et al. 1979; Ben-Avraham 1985). Due to trans- tensional plate motion (e.g. Garfunkel 1981), subsidence is taking place on the basin-bounding faults and transverse Fig. 1 a  Regional tectonic setting of the Dead Sea rift and  b generalized tectonic setting of the Gulf of Eilat/Aqaba showing themain transform fault segments and the three deep basins (after Ben-Avraham 1985). The traversing of the main transform fault to the west north of the Eilat subbasin was suggested by Ben-Avraham and Tibor (1993) and Ehrhardt et al. (2005).  c  Geological map (UTMcoordinates) of the northern Gulf of Eilat/Aqaba and southern AravaValley after Garfunkel et al. (2000). Main geological units:  y  Eilat granite (Precambrian),  yY   Yutim granite (Precambrian),  nqc  conglom-erate, undivided (Neogene-Quaternary),  q  alluvium (Holocene),  qp  playa deposits (Quaternary). Offshore, the ship tracks of the marinegeophysical survey are shown as  black lines . Note the denser track line grid in the tectonically active north-western sector of the gulf Geo-Mar Lett   faults at the ends of the basins. Tectonic uplift of the gulf margins is also evident in the flanking Precambrian graniticrocks that rise to over 1,000 m.The northernmost basin of the Gulf of Eilat/Aqaba,called the Eilat Deep, reaches a depth of about 900 mbsl(metre below sea level). Seismological data (Klinger et al.1999) indicate that the main left-lateral displacement in the1995 Nuweiba earthquake was accommodated along athrough-going strike-slip fault between the Aragonese Deepand the Eilat Deep (Fig. 1b). This is in agreement with thefault model of the northern gulf proposed by Ben-Avrahamand Tibor (1993), who interpreted the eastern boundaryfault of the Eilat Deep as the main strike-slip fault and thewestern boundary fault as a predominantly normal fault with minor strike-slip motion (Fig. 1b).The transition from the Eilat Deep to the AravaValley (Wadi  ’ Arabah), the adjacent continental sedi-mentary basin, takes place at the gulf head. Ben-Avraham and Tibor (1993) and Ehrhardt et al. (2005) suggested that the Eilat Deep basin is bounded in the north by a transverse fault, which transfers the major part of transform deformation from the eastern Eilat Deep to thewestern margin of the gulf head area. North of the gulf, theDST is traced along the Arava Valley. Surface mappingaided by aerial photography (Zak and Freund 1966; Garfunkel 1981; Ginat et al. 1998; Klinger et al. 2000a, 2000b; Niemi et al. 2001), and recent geophysical studies (Haberland et al. 2007; ten Brink et al. 2007) have revealed that the DST traverses the southern Arava Valley predominantly as a single, almost continuous, sinistralstrike-slip fault.The structure of the gulf head and the transition fromthe marine to continental basin is poorly understood. Thegulf narrows at this northernmost tip to approx. 6 kmwidth  —  a third the average width to the south. Severalearlier studies have mapped the bathymetry and seafloor morphology of the western margin of the gulf head (e.g.Reches et al. 1987; Ben-Avraham and Tibor  1993; Makovsky et al. 2008). To date, however, there has beenno work imaging the seafloor across the entire northernGulf of Eilat/Aqaba, spanning both Israeli and Jordanianterritorial waters.Within the context presented above, the purpose of this study is to report new findings and interpretations of seafloor morphology across the entire width of the gulf head, based on a high-resolution multibeam and side-scan sonar survey spanning the northern Gulf of Eilat/ Aqaba adjacent to the cities of Eilat, Israel and Aqaba,Jordan. Mapping the potentially active faults in theoffshore is an important component in defining theseismic hazards in this heavily populated region. Fur-thermore, gravitational slides on the seafloor would present a potential tsunami hazard. Materials and methods A marine geophysical survey was conducted in the northernGulf of Eilat/Aqaba at water depths of 10  –  700 m from 29October to 21 November 2006, onboard the Israel Ocean-ographic and Limnologic Research vessel R/V  Etziona .More than 280 multibeam lines were collected with a totallength of 400 km (Fig. 1).The multibeam data were gathered by means of a hull-mounted Simrad EM 1002 multibeam sonar system that operates at a frequency of 95 kHz and generates 111 2° beams spread over an arc of up to 150°. The spatialcoverage of the survey area gave a 60% overlap of themultibeam swaths, resulting in a spatial resolution of approx. 1-2 m. The sound velocity in the water columnwas measured with an accuracy  ∼ 0.05 m/s several times aday, using an Applied Microsystems Ltd. instrument that also recorded pressure, temperature and salinity. In addi-tion, we randomly checked the sound velocity values bycalculating the velocity (e.g. Mackenzie 1981) from thetemperature, pressure and salinity data. These data wereintegrated into the multibeam acquisition system to ensure proper water depth calculations.The multibeam data were processed to produce a 2-mdata grid of water depth measurements by means of threesoftware packages. The CARIS HIPS/SIPS software wasused to correct the raw data. Using the IVS Fledermaussoftware, we analyzed the spatial bathymetric data based ontwo- and three-dimensional visualization. This data pro-cessing includes integration of the velocity depth profiles,as well as data correction for survey positioning, heading,heave offset, pitch, roll and tides. The third software package used was the Geocoder (Fonseca and Calder 2006), which served for backscatter processing and analy-ses. A 1:20,000-scale map and a history of bathymetricsurveys in the Gulf of Eilat/Aqaba have been published inSade et al. (2008). A continuous land  –  sea, 20-m digital terrain model(DTM) grid was produced (Fig. 2) by merging the newmultibeam data with lower-resolution Hydrosweep datacollected outside the present survey area during the R/V  Meteor   cruise M44/3 in 1999 (Ehrhardt et al. 2005), and with sounding from existing bathymetric maps (e.g. Halland Ben-Avraham 1978). On land we used the 25-m DTMgrid of Israel and the 90-m SRTM-3 grid of Jordan. Theland morphology and its continuation to the offshore werecreated by combining a satellite image with the bathymetry.A 2.5-m false colour image (e.g. vegetation in red) from theFrench SPOT-5 satellite on 31 July 2006 was combinedwith the pseudo-colour 2-m resolution multibeam imagethat was created in this study (Fig. 3a).Side-scan sonar data were collected with an E-Sea Scan800 from Marimatech that was deep towed close to the Geo-Mar Lett   seafloor over selected target areas. In shallow water, theside-scan sonar was towed behind the R/V  Danny boy.  TheE-Sea Scan 800 was operated at 325 kHz with a rangereaching 150 m and a resultant estimated resolution of  ∼ 0.5  –  1 m. Processing was by means of CARIS software,including the following steps: converting the raw data,cleaning auxiliary sensor data, recomputing towfish naviga-tion, correcting slant range, generating mosaics, digitizingmorphological features such as reefs and, finally, importingthe resulting mosaics into the GIS database. Results Basin morphologyThe gulf head is 8.5 km long and 5  –  8 km wide ( ∼ 53 sq. km), bounded by the onshore Eilat Fault and the submarine AqabaFault. Profiles across this area and extending about 1 km onland clearly show the basin morphology (Fig. 3b). The gulf head is asymmetric towards the east. In the middle of thegulf, a north-south-trending bathymetric high  —  the Aylahigh  —  divides the basin into the Eilat subbasin to the west and the Aqaba subbasin to the east. The Ayla high isobserved from the mid-shelf ( ∼ 50 m isobath) to the deep basin ( ∼ 600 m isobath), and is about 6 km long and 1 kmwide. In some places it rises up to 50 m above thesurrounding seafloor (profiles F and G).Shelf   –  slope geometryIn Fig. 4, variations in the shelf   –  slope geometry of the Eilat and Aqaba subbasins are visible in the bathymetric map andin the 12 bathymetric profiles drawn perpendicular to thecoastline to a water depth of 300 m (cf. Table 1). Thenorthern termination of the gulf head is marked by astraight, northwest-trending shoreline. Modern reefs pres-ently grow only at the NW and NE corners at water depthsof about 15  –  30 m. In some places along the coast inshallow water, side-scan sonar images and backscatter analyses indicate that the seafloor is laden with coarsesediment deposits from the outwash of drainages to thenorth and east (Fig. 5). On the inner shelf parallel to thenorthwest shore of the gulf, live reefs are found generally to50 m water depth.Across the broad northeast shelf of the gulf head, bathymetric profiles clearly illustrate that, in contrast tothe western Eilat side, the shelf narrows by nearly 600 mon the eastern Aqaba side (Fig. 4, profiles 4  –  8). The shelf of the Aqaba subbasin (profiles 6  –  8) has a width of about 1,300 m and slopes of mostly 4  –  4.5°. The shelf   –  slope break occurs between water depths of 70  –  80 m and trends parallelto the shoreline. Continuous marine terraces, which canserve as proxies for local sea level at the time of their formation, are absent, indicating erosion and landwardretreat of the shelf. The slope is relatively steep (10  –  13°)and dissected by numerous shallow canyon heads with few Fig. 2  Colour three-dimensional regional elevation image of thenorthern Gulf of Eilat/Aqaba. The combined land  –  sea 20-m DTM was produced by merging the new multibeam data of the present studywith lower-resolution Hydrosweep data ( arrow A ) from outside the present survey area (Ehrhardt et al. 2005) and with sounding fromexisting bathymetric maps (e.g. Hall and Ben-Avraham 1978;  arrow B ). On land, the data include the 25-m DTM grid of Israel and the 90-m SRTM-3 grid of JordanGeo-Mar Lett   displaced terraces. In the Eilat subbasin, the width of theshelf increases to the west from 1,400 m to 1,800 m and hasslope angles of mostly 3  –  3.5°. The shelf   –  slope break trendsobliquely ( ∼ 55° anticlockwise) to the present coastline andis marked by a terrace located at about 100 m. The slope isdissected by one major canyon system that we named theEilat Canyon (profile 4). The Ayla high separates the basinstarting at 50 m water depth (profile B in Fig. 3b, profile 5in Fig. 4b).The shelf   –  slope geometry on the western side of theEilat subbasin (profiles 1  –  3 in Fig. 4b) is characterized byliving reefs on the inner shelf that extend 300 m offshore to50 m water depth (Fig. 5). In several places, living reefs arefound to more than 80 m water depth. The width of theshelf decreases to the northwest from about 810 m to 380 mwith slope angles of mostly 6° and 13° respectively. Theouter shelf and slope change their character near the latitudeof the Shlomo Canyon (Fig. 6). North of this, the shelf   –  slope break is defined by a distinct terrace at water depths between 95  –  100 m and is marked by slump scars (profile 3in Fig. 4b). South of this latitude, distinct ancient terraceswith fossil reefs can be seen at water depths of 120 and140 m, and several slump scars are found along thesouthern part of the slope. Fig. 3 a  Multibeam image map of the northern Gulf of Eilat/Aqabashowing 20 m isobaths and the locations of profiles  A  –   J   across the basin. A false colour SPOT 5 image of 31 July 2006 with a pixelresolution of 2.5 m is displayed onshore.  b  Land  –  sea profiles  A  –   J  showing the asymmetry of the basin, and its subdivision into threemain structural elements: the Eilat and Aqaba subbasins, separated bythe slightly southeast-tilted Ayla high.  V.E.  Vertical exaggerationGeo-Mar Lett 
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