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A model for the Hellenic subduction zone in the area of Crete based on seismological investigations

A model for the Hellenic subduction zone in the area of Crete based on seismological investigations
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  doi:10.1144/SP291.9 2007; v. 291; p. 183-199 Geological Society, London, Special Publications   T. Meier, D. Becker, B. Endrun, M. Rische, M. Bohnhoff, B. Stöckhert and H.-P. Harjes based on seismological investigationsA model for the Hellenic subduction zone in the area of Crete   Geological Society, London, Special Publications   serviceEmail alerting article to receive free email alerts when new articles cite thisclick here  requestPermission  to seek permission to re-use all or part of this article click here  Subscribe Publications or the Lyell Collection to subscribe to Geological Society, London, Specialclick here  Notes   Downloaded by Stanford University on 20 December 2007 London  © 2007 Geological Society of  A model for the Hellenic subduction zone in the area of Cretebased on seismological investigations T. MEIER 1 , D. BECKER 1 , B. ENDRUN 1 , M. RISCHE 1 , M. BOHNHOFF 2 ,B. STO¨CKHERT 1 & H.-P. HARJES 1 1  Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, NA 3 / 173,Universita¨ tsstr. 150, D-44780 Bochum, Germany (e-mail: 2 GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germany Abstract:  The island of Crete represents a horst structure located in the central forearc of theretreating Hellenic subduction zone. The structure and dynamics of the plate boundary in thearea of Crete are investigated by receiver function, surface wave and microseismicity using tem-porary seismic networks. Here the results are summarized and implications for geodynamicmodels are discussed. The oceanic Moho of the subducted African plate is situated at a depthof about 50–60 km beneath Crete. The continental crust of the overriding Aegean lithosphereis about 35 km thick in eastern and central Crete, and typical crustal velocities are observeddown to the upper surface of the downgoing slab beneath western Crete. A negative phase atabout 4 s in receiver functions occurring in stripes parallel to the trend of the island points tolow-velocity slices within the Aegean lithosphere. Interplate seismicity is spread out about100 km updip from the southern coastline of Crete. To the south of western Crete, this seismicallyactive zone corresponds to the inferred rupture plane of the magnitude 8 earthquake of   AD  365. Incontrast,interplatemotionappears tobe largely aseismicbeneaththe island.The coastlineof Cretemimics the shape of a microseismically quiet realm in the Aegean lithosphere at 20–40 km depth,suggesting a relation between active processes at this depth range and uplift. The peculiar prop-erties of the lithosphere and the plate interface beneath Crete are tentatively attributed to extrusionof material from a subduction channel, driving differential uplift of the island by several kilo-metres since about 4 Ma. Since the Late Cretaceous, the tectonics of theEastern Mediterranean region has been controlledby (1) convergence between Africa and Eurasiaand (2) subduction of oceanic lithosphere of narrow small oceanic basins separating Gondwana-derived terranes (e.g. Dercourt  et al.  1986; Gealey1988; Stampfli & Borel 2004). Oceanic lithosphereis now almost completely subducted, with remain-ing remnants beneath the Ionian basin and in theEastern Mediterranean south of western Turkey.The closure of the earlier subducted oceanicbasins and the collision of the intervening terraneswith the Eurasian active margin resulted in anumber of distinct orogenic belts and accretion of continental crust. In the Aegean, rollback of theactive continental margin has been importantsince at least 30 Ma (e.g. Angelier  et al.  1982;Thomson  et al.  1998; ten Veen & Postma 1999),collision between the African passive margin andthe Aegean continental lithosphere is incipient(Mascle & Chaumillon 1997; Mascle  et al.  1999;ten Veen & Kleinspehn 2003; Meier  et al.  2004 a ).The position of subducted oceanic lithosphere inthe Hellenic subduction zone was imaged by seismictomography in the upper mantle (Spakman  et al. 1988; Ligdas  et al.  1990; Papazachos  et al.  1995;Taymaz 1996; Alessandrini  et al.  1997; Papazachos& Nolet 1997; Piromallo & Morelli 1997, 2003;Marone  et al.  2004) and well into the lowermantle (Spakman  et al.  1993; Bijward  et al.  1998;Bijward & Spakman 2000; Karason & van derHilst 2000; Faccenna  et al.  2003), down to depthsof about 1700 km. Only the upper part of the slabrepresents African lithosphere that was subductedat the present active continental margin south of Crete. Subduction south of Crete started at about20–15 Ma, when the plate boundary shifted to thesouthern border of an accreted microcontinentbuilding most of the continental crust of presentCrete (Thomson  et al.  1998). The active marginhas since retreated by about 350–500 km in asouthwestward direction (ten Veen & Meijer1998; ten Veen & Kleinspehn 2003). The coevalconvergence between the African and the Eurasianplates amounts to about 150 km (Dercourt  et al. 1986; Gealey 1988; Jolivet & Faccenna 2000;Faccenna  et al.  2003; Stampfli & Borel 2004).This means that a slab of about 500–650 kmlength entered the subduction zone since the plateboundary shift to the south of Crete (Meier  et al. 2004 a ). In accordance with tectonic reconstructions(Dercourt  et al.  1986; Gealey 1988; Stampfli & From : T AYMAZ , T., Y ILMAZ , Y. & D ILEK  , Y. (eds)  The Geodynamics of the Aegean and Anatolia. Geological Society, London, Special Publications,  291 , 183–199.DOI: 10.1144 / SP291.9 0305-8719 / 07 / $15.00 # The Geological Society of London 2007.  Borel 2004), the slab of more than 2000 km lengthimaged by seismic tomography must be composedof lithosphere once underlying different oceanicbasins that were successively subducted at differentactive margins. After closure of each oceanic basinand collision of the trailing continental terrane withtheEurasianmargin,anewactivemargindevelopedto the south of the accreted terrane, and subductionof the next stripe of oceanic lithosphere com-menced. To explain the continuous slab imaged byseismic tomography, Meier  et al.  (2004 a ) proposedthat continental crust of the terranes was delami-nated from the underlying continental mantle litho-sphere, as proposed by Thomson  et al.  (1998, 1999)in the ‘buoyant escape’ scenario based on the geolo-gical record of Crete. If this is true, the slab imagedbyseismictomographywouldbecomposedofalter-nating sections of oceanic and continental mantlelithosphere (Meier  et al.  2004 a ; van Hinsbergen et al.  2005) of variable width, corresponding to theoceanic basins and continental terranes depicted inthe palaeogeographical reconstructions (Dercourt et al.  1986; Gealey 1988; Stampfli & Borel 2004).It is possible that the presence of sections of delami-nated subcontinental mantle devoid of hydratedmaterial is the reason for the missing deep seismi-city in the present-day Hellenic subduction zone.The recent kinematics of the Eastern Mediterra-nean are characterized by westward drift andcounter-clockwise rotation of the Anatolian micro-plate and southward escape of the Aegean litho-sphere (McKenzie 1970, 1972, 1978; Le Pichon et al.  1995; McClusky  et al.  2000). Global position-ing system (GPS) surveys demonstrate that the rateof SSW motion of the Aegean lithosphere withrespect to stable Eurasia increases towards thesouth (McClusky  et al.  2000). This implies exten-sion in the overriding plate, which is attributed torollback and retreat of the subduction zone (LePichon & Angelier 1979; Angelier  et al.  1982; tenVeen & Meijer 1998; Armijo  et al.  2003). Withinthe given kinematic framework, the high curvatureof the active continental margin implies that sub-duction becomes increasingly oblique from west toeast. In the western forearc, the vectors of motionare oriented normal to the continental margin.Towards the east, in the area of Rhodos, the anglebetween the motion vector and the active margindecreases to about 20º. The relative velocitybetween the subducting African plate and the over-riding Aegean lithosphere is about 4.5 cm a 2 1 (McClusky  et al.  2000) at the active margin.Seismic activity in the forearc of the Hellenicsubduction zone is considerable. In the last 2.5 kaseveral events reached a magnitude of eight(Papazachos  et al.  2003). In  AD  365, an eventwith an estimated magnitude of 8.3 occurred inthe western forearc, to the SW of Crete. The lateHolocene uplift, of up to 9 m at the southwesterncoast of Crete, is attributed to co-seismic defor-mation during this event (Pirazzoli  et al.  1982;Stiros 2001). According to the global catalogue of Engdahl  etal.  (1998), eight events witha magnitudeof six or above occurred in the Hellenic subductionzone in the second half of the 20th century.One peculiarity of the Hellenic forearc is thegeologically recent rapid uplift in the area of Crete (Meulenkamp  et al.  1994; Lambeck 1995),of up to several kilometres since about 4 Ma withan average rate close to 1 mm a 2 1 . This Plio-Pleistocene uplift has produced a marked relief,with Crete rising to nearly 2.5 km above sea level,close to narrow furrows with water depths of more than 3 km to the south. These deep topo-graphic furrows to the south of Crete do not mark the position of the plate boundary (Le Pichon &Angelier 1979; Angelier  et al.  1982). The widelyused term ‘Hellenic trench’, commonly used torefer to the distinct topographic feature markingthe site where the subducted plate disappearsbeneath the forearc at a convergent plate boundary,may be misleading here. In the Hellenic subductionzone, the accretionary complex (e.g. Kastens 1991;Mascle & Chaumillon 1997) is located to the southof these ‘trenches’ beneath the Mediterranean ridge,and the trenches mark tectonic features within theforearc, and hence within the overriding Aegeanlithosphere. The Cretan horst structure shows thefollowing characteristic features: (1) it is narrowin the north–south direction; (2) it is bounded bynormal faults on all sides; (3) uplift commencedabruptly in the early Pliocene; (4) uplift is contem-poraneous with continuing forearc-parallel exten-sion of the Aegean lithosphere. The question is:what drives Crete up?In this paper, recent results of receiver function,surface-wave and microseismicity studies usingtemporary local networks are reviewed. The impli-cations for the structure and dynamics of the Helle-nic subduction zone are discussed and a model forthe rapid localized uplift of Crete is proposed. Slab segmentation in the Hellenicsubduction zone The seismicity of the Hellenic subduction zonebetween1964and1998accordingtotheglobalrelo-cated ISC catalogue (Engdahl  et al.  1998) is shownin Figure 1. In this region the catalogue is completefor events with magnitudes greater than  c . 4.7. Clus-ters of events are observed within the part of theforearc characterized by intense shallow seismicity.The trend of these clusters is oriented NE–SW(Fig. 1). The depth of hypocentres increases withdistance from the plate boundary and reaches T. MEIER  ET AL. 184  about 100 km in the west and about 160 km inthe east. These intermediate depth events define astrongly curved Benioff zone, with its lower ter-mination approximately beneath the volcanic arc(Papazachos & Comninakis 1971; Comninakis &Papazachos 1980; Makropoulos & Burton 1984;Taymaz  et al.  1990; Hatzfeld & Martin 1992;Knapmeyer 1999; Papazachos  et al.  2000).The hypocentres of the relocated ISC catalogue(Engdahl  et al.  1998) are projected onto verticalcross-sections in Figure 2. The figure shows thatthere is not much intermediate-depth Benioff zoneseismicity compared with, for example, thewestern Pacific subduction zones. As is evidentfrom Figure 2d and e, the Benioff zone can betraced to a depth of about 160 km, with a faintseismic gap between about 100 and 125 km(Papazachos  et al.  2004). In the east, the Benioff zone dips more steeply than in the west. As isevident from Figure 2, data from local networksare required to image the seismogenic zonesbeneath the forearc in greater detail. Fig. 1.  Hypocentres of events in the Aegean region listed in the relocated ISC catalogue (Engdahl  et al.  1998) for theyears 1964–1998. The position of the deformation front of the Hellenic subduction zone (Le Pichon  et al.  1995)and the border between the central and the inner units of the Mediterranean Ridge (continuous line with triangles,Lallemant  et al.  1994; dashed–dotted line with triangles, Mascle  et al.  1999) are indicated. Points labelled withnumbers mark the southern border of the Aegean crust as determined by seismic studies (1, Lallemant  et al.  1994; 2,Bro¨nner 2003; 3, Bohnhoff   et al.  2001), corresponding to the transition between the inner and the central units of the Mediterranean Ridge. Relative horizontal velocities with respect to Eurasia (McClusky  et al.  2000) are indicated byarrows. In addition, the deep furrows (‘trenches’) to the south of Crete and the NE–SW-striking linear patterns of seismicity in western and central Crete are emphasized.A MODEL FOR THE HELLENIC SUBDUCTION ZONE 185  In the western Cyprus arc (section f in Fig. 2),the hypocentres define a Benioff zone with subduc-tion towards the NE beneath Anatolia. As the slab inthe eastern part of the Hellenic subduction zone isinclinedtowards the NW, a slab windowis expectedto exist beneath western Turkey, which probablyaffects deformation and magmatism of this region.The schematic sketch in Figure 3 illustrates the situ-ation, as suggested by intermediate-depth seismi-city (e.g. Engdahl  et al.  1998).In the western segment of the Hellenic subduc-tion zone, the dip of the slab increases at a depthof about 75 km (Hatzfeld 1994; Papazachos  et al. 1995, 2000; Taymaz  et al.  1990; Papazachos &Nolet 1997). Based on a regional tomographicstudy, Papazachos & Nolet (1997) concluded thatat about 200 km depth the western and easternslab segments meet along a rather sharp edge witha right angle. Wortel & Spakman (2000) assumeda tear within the slab, propagating horizontallyfrom the Ionian Sea along the forearc, whichcauses incipient slab detachment. Apart from thestrong curvature enhanced by rollback (e.g. tenVeen & Kleinspehn 2003), the segmentation of  Fig. 2.  Hypocentres from the global catalogue by Engdahl  et al.  (1998) projected onto vertical planes along the profilelines indicated in the map.T. MEIER  ET AL. 186
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