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Anatomy of the North Anatolian Fault Zone in the Marmara Sea, Western Turkey: Extensional Basins Above a Continental Transform

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Anatomy of the North Anatolian Fault Zone in the Marmara Sea, Western Turkey: Extensional Basins Above a Continental Transform Anatomy of the North Anatolian Fault Zone in the Marmara Sea, Western Turkey: Extensional Basins Above a Continental Transform Ali E. Aksu, Tom J. Calon, Richard N. Hiscott, Department of Earth Sciences, Centre for Earth Resources Research, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada, [email protected] Dog˘an Yas¸ar, Institute of Marine Sciences and Technology, Dokuz Eylül University, Haydar Aliyev Caddesi No. 10, Inciraltı, Izmir 35340, Turkey ABSTRACT INTRODUCTION Although it straddles an area of extreme earthquake risk, In the January issue of GSA Today, Reilinger et al. (2000) the origin of the Marmara Sea transtensional basin has been explained the inevitability of destructive earthquakes along the enigmatic. Recently acquired high-resolution seismic profiles North Anatolian transform fault of northern Turkey as a conse- and earthquake hypocenter locations show the crustal archi- quence of the westward tectonic escape of the Aegean-Anatolian tecture to be characterized by a negative flower structure, Plate from a collision zone between the converging African and bounded by two west-trending sidewall faults that are linked Eurasian plates (Fig. 1, inset). They pointed to the lack of a to a single vertical to steeply south-dipping master fault that detailed map of faults crossing the locally deep (>1200 m) floor of extends to depths of >30 km. The negative flower structure the Marmara Sea (Fig. 2A) as an impediment to establishing the has a complicated architecture consisting of relatively intact precise mechanics of faulting and earthquake generation. This detached basinal blocks, separated by southwest-trending region is of critical concern because devastating earthquakes over ridges which serve as strike-slip transfer zones between the the past 100 years have progressed westward along the plate basins. The basins and ridges are rotating counterclockwise, boundary toward the Marmara Sea region (Reilinger et al., 2000). accommodated by the southward retreat of the southern Because of poor constraints on fault geometry, conflicting tec- sidewall of the flower structure as crustal material is passed tonic interpretations have been proposed for the deep basins of from its eastern to western end along the transtensional the Marmara Sea and associated seismicity (Fig. 2C and 2D). strike-slip zone. This new interpretation provides a better Comparisons of existing models show that separate groups of context for understanding seismicity in the region and for authors have advocated different locations for fundamental understanding complexities of fault segmentation in large transtensional basins along continental transforms in zones Fault Zone, Turkey continued on p. 4 of tectonic escape. Figure 1. Structural map Black Sea EURASIAN PLATE B Basin 2 B' A Basin 5 A' showing margins of principal Northern Anatolian Transform deformation zone (PDZ), ° Basin 3 major strike-slip faults (half 40 N ~10 km Marmara Sea arrows), and normal faults with ticks on hanging wall. AEGEAN-ANATOLIAN PLATE ~10 km α β γ δ ~6:1 vertical Areas , , , and are major exaggeration ~6:1 vertical ridges. Structural features are 36°N exaggeration taken directly from interpreted seismic profiles along closely spaced survey tracks (Fig. 2E), Mediterranean Sea ARABIAN Bosphorus so this figure is just like the AFRICAN PLATE PLATE 32°N Buried master field map of a land geologist. 20 mm/yr fault Northern The only interpreted features 20°E 30°E 40°E β γ margin of PDZ º are buried, dashed trace of B dextral δ Izmit 41 N dextral North Anatolian trans- normal releasingA Bay form fault (NATF) and inferred zones of compression (re- straining bends) and exten- sion (releasing bends). Upper restraining? left inset is simplified tectonic map of eastern Mediterranean Saros-Ganos releasing conservativeFault region, showing sense of plate B' Yalova Fault motion (large gray-headed α Southern margin Side-wall faults arrows) and global positioning of PDZ system (GPS) horizontal veloc- A' of PDZ ities of Aegean-Anatolian plate Major strike-slip (from Reilinger et al., 2000) faults relative to a fixed Eurasian Scale (km) Normal fault plate (thin black arrows scaled 02040 AEGEAN-ANATOLIAN Rollover anticline Ridge crest in length to GPS velocities in º º mm/yr). Half arrows indicate 28 E PLATE 29 E transform or strike-slip faults. Cross sections A–A' and B–B' show our perception of architecture of elongate negative flower structure where it has central anticlinal swell and step-out basin perched on edge of principal deformation zone (A–A'; compare Fig. 4A), and where it encloses symmetrical graben (B–B'; compare Fig. 4B). Green substratum in cross sections represents older deposits beneath Pliocene to Quaternary basin fill. GSA TODAY, June 2000 3 28°E 29°E 0 0 20 6040 10 A Scale (km) Buried master fault Basin 4 Basin 1 Basin 2 20 Bosphorus 50 ° 100 41 N Depth (km) B 50 extension 27°51' - 27°57'E 400 50 Basin 3 strike- 30 1200 slip 41°N 800 Latitude 1000 1200 Izmit 600 400 600 200 Bay 100 400 Depth of fault 100 Hypocenter Ganos Basin 5 1-10 km 50 50 50 11-20 km 50 100 21-30 km Dardanelles >30 km Wong et al. (1995), Okay et al. (1999) Survey Fig. 4B Ergün & Özel (1995) tracks Fig. 4A C D Seng, ör et al. (1985, fig. 13) E Figure 2. A: Bathymetry simplified from Aksu et al. (1999), depths of hypocenters of selected 1970–1998 earthquakes (Bog˘aziçi University, Kandilli Geological Observatory; see koeri.boun.edu.tr), margins of principal deformation zone (PDZ, thick purple lines), and position of steeply dipping dextral master fault (thick pink lines) to west of Marmara Sea (= Saros-Ganos fault) and as dashed line where buried beneath elongate negative flower structure along axis of principal deformation zone. Of five principal basins, four straddle principal deformation zone and lie directly above buried North Anatolian transform fault. Soccer- ball–shaped symbols are lower-hemisphere projections of fault-plane solutions for selected large earthquakes (compiled by Kiratze and Papazachos, 1995; Wong et al., 1995; Yalıtırak et al., 1998); quadrants with compressional first motion are black. B: Cross section along north-south line at ~27.6°E in A, showing distribution of earthquake hypocenters in band of latitudinal width 0.06', projected into cross section. Other cross sections are essentially identical and reveal location and dip of buried master fault (= North Anatolian transform fault) beneath principal deformation zone. Patterned region from 0 to 5 km depth is out- line of principal deformation zone from seismic displays of Okay et al. (1999). Note lack of correlation between earthquake hypocenters and sidewall faults of principal deformation zone. C and D: Contradictory fault patterns proposed for Marmara Sea by various authors. E: Survey tracks of seismic grid used to con- struct Figure 1 and locations of seismic profiles of Figure 4. Fault Zone, Turkey continued from p. 3 et al., 2000), demonstrating a counter- transform-parallel strike-slip basin (Fig. clockwise rotation of the Aegean- 3B), and shows that it is instead a rather strike-slip faults, contrasting asymmetries Anatolian plate and a progressive south- unconventional negative flower structure for adjacent strike-slip basins, and differ- westward increase in plate velocity in the with complex internal geometry (Fig. 3C). ent linkages with faults on land. This high Aegean region (Fig. 1, inset). Mann (1997) formulated a general model level of uncertainty as to the first-order Published tectonic models have failed for the formation of large transtensional geometry of structures makes it impossible to properly explain the origin of the basins in zones of tectonic escape empha- to confidently evaluate the seismicity of Marmara Sea because of poor seismic sizing the hybrid nature of such basins in the Marmara Sea area. coverage and insufficient use of available terms of both pull-apart and transform- The Marmara Sea region is also an earthquake data. For example, cross- normal extensional styles. We believe that important place for understanding the sectional plots of the locations of earth- this notion is directly applicable to the nature of transform plate boundaries. The quake hypocenters beneath the deeper Marmara Sea. North Anatolian transform fault forms areas of the Marmara Sea (Fig. 2B) show the northern boundary of the Aegean- that the steep marginal fault scarps enclos- BATHYMETRY Anatolian plate and accommodates its ing the deep basins are not fundamental Bathymetry provides a first-order data westward escape by dextral strike-slip crustal-scale faults (i.e., none of these are set for inferring the positions of surface movement (Fig. 1, inset). The Marmara main strands of the North Anatolian faults, the geometry of uplift and subsi- Sea is located on the transform fault, at transform fault). Instead, the plate bound- dence, and the interaction of faulting and a place where a notable southwestward ary fault lies directly beneath the axis of sedimentation. The Marmara Sea is a swing occurs in the velocity field of the the Marmara Sea, where it is buried by a 30–35-km-wide and 150-km-long, west- Aegean-Anatolian plate and where a broad structurally complex zone of rhombohe- trending depression that consists of steep- zone of faults swings gradually to the dral to elongate basins and ridges. This flanked basins and ridges (10°–30° slopes) southwest to connect the North Anatolian observation, combined with new maps of nestled between a 3–5-km-wide shelf dom- transform fault to the Saros-Ganos fault bathymetry (Fig. 2A) and fault traces (Fig. inated by eroded Tertiary bedrock in the (Figs. 1 and 2). Global positioning system 1) that we have prepared from closely north and an ~30-km-wide shelf in the measurements constrain the horizontal spaced seismic profiles (Fig. 2E), allows us south (Fig.
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