Genphys. J. Int. (1997) 131,61-86
Recent temporal change in the stress state and modern stress field along the North Anatolian Fault Zone (Turkey)
Olivier Bellier,' Semir Over,2 Andrk Poisson' and Jean Andrieux' URACNRS 1369, Giophysique et Giodynamique Interne, Bit. 509, University of Paris Sud, 91405 Orsay Cedex, France. E-mail: [email protected] Department of Geology, Cumhuriyet, University ofSivas, 58140 Sivas, Turkey
Accepted 1997 April 30. Received 1997 January 31; in original form 1996 February 14 Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021
SUMMARY The North Anatolian Fault (NAF) is a dextral strike-slip fault which runs about 1400 km from east to west, from the Karliova triple junction to the Aegean domain. It was active during the Plio-Quaternary as a consequence of the collision between the Eurasian and Arabian plates. The NAF intracontinental deformation zone contributes with the sinistral East Anatolian Fault to the westward extrusion of Anatolia as a consequence of the northward drift of Arabia. The Central NAF zone forms a northward convex bend where the eastern NAF N1 10"E-trending segment extending from Karliova is connected to the western NAF N75"E-trending segment running to Aegea. Analysing the slip occurring in major earthquakes around the NAF, we show that the present-day stress pattern along the NAF agrees with stress magnitude and strike variations from east to west. The stress regime along the NAF is NNW-trending aHmax(al) transpression in eastern NAF; this progressively changes westwards to NW-trending c, transtension in central Anatolia. This lateral variation means a relative decrease of the aHmaxmagnitude, which evolves progressively to an extensional regime (a, vertical) around the westernmost NAF, characterized by a NNE-trending chmin (a3). The drastic change in the tendency of the regional stress state in the North Anatolian block, from compressional to extensional, is effective within the central NAF bend. The stress regime determined by inversion of slip vectors measured on fault planes of various scales confirms the present-day transtensional regime in the central part of the NAF bend. However, the fault kinematic analysis of Quaternary stress states within the Central NAF indicates that this stress state was not continuous throughout the Quaternary. Indeed, chronologies of fault slip vectors provide evidence for two distinct Quaternary regional strike-slip stress states within this zone. Both states have consistent NE- and NW-trending a3 and a1 axes, respectively, but have significantly different R values. The change in the strike-slip stress regime probably occurred in the middle Pleistocene. The older mean stress state is characterized by a N142 f 8"E- trending c,, a N52 f 13"E-trending a3 and a mean arithmetic R, value of 0.75, which indicates that the regional stress regime is transpressional. The younger strike-slip regime is characterized by a N142" f 14"E al axis, a N52" f 10"E a3 axis and a mean R, of 0.24, which indicates the transtensional character for this regime. The low R values of the stress deviators related to the recent stress state reflect normal- component slips. These temporal and spatial stress changes along the NAF result from the coeval influence of forces due to the Aegean subduction in the west and to the northward drift of Arabia in the east. Over these boundary forces, shear is superimposed along the NAF, which accommodates the Anatolia extrusion. However, the timing of the temporal stress change permits the suggestion that the Quaternary stress regme variation in North Anatolia is mainly due to the influence of the Aegean domain. Key words: Fault slip, inversion, North Anatolian Fault, Quaternary, stress distribution, strike-slip.
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stress fields since late Miocene-early Pliocene times. Although INTRODUCTION the NAF is one of the world's major active strike-slip fault The North Anatolian Fault (NAF) is one of the major active systems, and despite a century of active studies (see Ambraseys structures in the eastern Mediterranean region. It is a dextral 1970; McKenzie 1972; Sengor 1979; Jackson & McKenzie strike-slip fault zone which runs about 1400 km from the 1984; Barka & Hancock 1984; Barka 1992), the Anatolian Karliova triple junction in the east to the Aegean extensional domain fault kinematics and stress-regime history are still domain in the west (Fig. 1) (e.g. McKenzie 1972; Sengor 1979; poorly known; the greater knowledge of the Cenozoic stress Dewey & Sengor 1979). The NAF roughly follows the trace regime mainly concerns the western domain of Anatoha (e.g. of a north-dipping subduction zone that was active during the Angelier et al. 1981; Hancock & Barka 1987; Zanchi & Angelier Late Cretaceous (Sengor & Yilmaz 1981). The NAF intraconti- 1993). To complement the NAF stress-regime knowledge, we nental deformation zone constitutes the northern boundary of studied the recent stress-regime changes of the NAF, particu- the Anatolian block, active since the Late Miocene (Dewey larly within the central part at the location of the northward et al. 1986), or more probably since the early Pliocene (e.g. convex bend (Fig. 1).In this domain, the eastern NAF N110"- Barka 1992; Westaway 1994), as a consequence of the collision trending segments extending from Karliova to the convex bend between the Eurasian and Arabian plates. In the east, the are connected to the western NAF N75"-trending segments Anatolian block is deformed within the eastern Anatolian running to the Aegean graben system. Constraints on the Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 convergence zone by roughly NNE-trending compression Cenozoic states of stress along the central NAF are provided induced by the northward collision of the Arabian plate by inversions of slip vectors measured on minor and major towards the Eurasian (e.g. McKenzie 1972; Jackson & McKenzie fault planes (Over et al. 1993) that affect the Mesozoic basement 1984; Philip 1987). At its westernmost extremity, the Anatolian and the Cenozoic basin sediments. This previous study in the block is deformed by the Aegean extensional regime (e.g. Le NAF convex zone provides evidence for Late Cenozoic signifi- Pichon & Angelier 1979; Dewey & Sengor 1979; Mercier et al. cant variations of the tectonic principal stresses, in both 1979; Mercier, Sorel & Simeakis 1987; Mercier, Sorel & Vergely orientation and magnitude. The current study complements 1989). Thus, as a first approximation, the strike-slip regime of the subset study of Cenozoic stress states within the central the central NAF is superimposed westwards on the Aegean NAF. We shall focus this study on two specific issues: (1) the extension and eastwards on a compressional regime. characterization of a spatial change of the present-day stress The coeval influences of the deep-seated extension of the field acting along the NAF by inversions of seismically deter- Aegean domain to the west, the compressional regime acting mined slip vectors provided by published earthquake focal to the east (east of the Karliova triple junction), and the mechanisms; (2) the identification of a temporal change of the superimposed shear related to the accommodation of the Quaternary stress regime in the central NAF zone by inversions extrusion displacement could produce complex and changing of slip vectors (striae) measured on fault planes of various scales.
Plate tectonic model .-1 IIc S.L.R. vector with respect to Eurasia Displacement rate d Rigid rotation best fitting vector -1
Figure 1. Sketch map of the Eastern Mediterranean tectonic framework (modified after Sengor 1979). Plate-tectonic-model vector results from NUVEL-1 (DeMets et al. 1990). The dashed circular arcs correspond to the Anatolian mass-transfer trajectory deduced from rigid-rotation modelling (Le Pichon et al. 1995). NAF indicates the North Anatolian Fault; DSTF, the Dead Sea Transform Fault; EAF, the East Anatolian Fault; and BTFB, the Bitlis Thrust and Fold Belt.
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The geodetically determined Anatolia-Aegea mean displace- GEODYNAMIC SETTING ment of 25 nim yr-' approximately corresponds to a rigid Collision and amalgamation of the continental fragments along rotation accommodating the transfer of Arabia motion to the southern margin of the Pontides in North Anatolia Anatolia (Le Pichon et al. 1995), a small part of this motion occurred between the Eocene and the middle Miocene. After being accommodated by continental collision, which produces that, the closure of the Bitlis oceanic basin, part of the southern earthquake activity and high topography along the Bitlis Thrust Tethys branch corresponding to the Bitlis suture, lead to and Fold Belt (Fig. 1). This suggests that forces applied at the collision between the Arabian and Eurasian plates (e.g. Sengor, Anatolia/Arabia boundary play a major role. However, even Gorur & Saroglu 1985). The northward convergence of Arabia if Turkey and Aegea are moving as a single plate (Oral et a/. thus induced a continental collision which produced crustal 1995), thoughtful reappraisal of available space-based measure- thickening in eastern Turkey and the Caucasus mountains, ments shows an approximately 10mm yr-' increase for the along the Bitlis Thrust and Fold Belt (Fig. 1). Thickening Anatolia extrusion rate towards the southwest, i.e. towards the is presently continuing as Arabia moves northwards with Hellenic arc (Oral et ul. 1995; Le Pichon et al. 1995), whereas respect to Anatolia along the Dead Sea transform fault. the overall pattern confirms crustal extension in Aegea and Contemporaneously, this northward Arabian drift is inducing western Turkey (Oral et al. 1995). The southwestward rate the westward extrusion of the Anatolian block, which is increase of the Anatolian extrusion suggests that the extrusion Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 bounded by the dextral NAF and the sinistral East Anatolian is facilitated by the Aegean extension in relation to the Hellenic Fault (McKenzie 1972; Tapponnier 1977; Sengor 1979; Sengor subduction. Therefore, the general westward movement of et al. 1985; Dewey et al. 1986). Anatolia results from forces applied at both of its extremities: Plate tectonic models (NUVEL-1, DeMets et al. 1990) the westward push and extrusion due to the northward Arabian indicate that the Arabian plate is moving in a north- drift to the east, and the southwestward pull from the Hellenic northwesterly direction relative to Eurasia at a rate of about subduction to the west. 25 mm yr-', while the African plate is moving northwards at a rate of approximately 10 mm year-' (Fig. l), the differential THE CENTRAL NAF CONVEX-BEND motion being taken up mainly by left-lateral displacement SETTING along the Dead Sea fault. The leading edge of the African plate is subducting along the Hellenic arc at a higher rate than the The present-day structural pattern of the NAF convex bend is relative northward motion of the African plate. This requires mainly inherited from pre-Neogene tectonics (Andrieux et al. that the Hellenic trench is moving southwards relative to 1995). The NAF follows through the Pontide Belt, along which Eurasia as a consequence of the roll-back of the Mediterranean Neogene to Quaternary basins were formed. In the convex slab, which is being subducted beneath the Aegean Sea. This bend, these basins are, from west to east, the Cerkez-Ilgaz, slab retreat has been attributed to the Mediterranean slab-pull Tosya, Kargi and Havza-Ladik (Fig. 2). They are elongate force, which produces a decrease of the horizontal stresses in basins that parallel the convex bend of the NAF, and were the overriding plate, i.e. widespread extensional tectonics infilled by volcanosedimentary deposits and then lacustrine in the Aegean domain and western Anatolia (e.g. Mercier et al. and fluvial sediments from the Miocene onwards (Irrlitz 1972; 1979,1989; Le Pichon 1982; Sorel et al. 1988). Crustal extension Barka & Hancock 1984; Barka & Gulen 1988; Over 1996). in this region is generally consistent with independent GPS Most of the basin infill is composed of lacustrine and fluvio- results (e.g. Oral et a/. 1995; Straub & Kahle 1995; Le Pichon lacustrine sediments, attributed to the Pontus Group, which is et a/. 1995), as well as with earthquake focal mechanisms and divided into two locally observed formations that are separated geological structure (e.g. McKenzie 1972, 1978; Jackson & by an angular unconformity: the Lower and Upper Pontus McKenzie 1984; Mercier et al. 1979,1989). In fact, this extension (Barka & Hancock 1984; Barka & Gulen 1988; Andrieux et al. has been interpreted as a consequence of a combination of 1995; Over 1996). These formations have been attributed to gravitational forces towards the Hellenic trench due to the the Tortonian (Late Miocene) and Plio-Pleistocene, respect- topography of Anatolia-Aegea, and the Mediterranean slab- ively (Barka & Hancock 1984). However, a critical reappraisal retreat force (e.g. Le Pichon 1982; Mercier et al. 1989; Sorel of the available stratigraphical data give a younger age for the et al. 1988; Meijer 1995). lacustrine mark of the Lower Pontus Formation in the Ilgaz Spatial geodetic measurements (GPS and SLR) have pro- and Tosya basins (Over 1996). In addition, Over (1996) vided parameters for the modern deformation of Anatolia and obtained a Pliocene age for the Lower Pontus Formation on Aegea (Noomen, Ambrosius & Wakker 1993; Oral et al. the basis of stratigraphically significant fresh-water charophyts 1993, 1995; Straub & Kahle 1994, 1995; Le Pichon et al. 1995). and ostracods. They indicate a mean extrusion rate of Anatolia with respect The ENE-trending Cerkes-Ilgaz Basin is located 5 km south to Europe of about 25 mm year-' along the NAF (Fig. 1). of the main NAF trace. A N70"E-trending south-verging thrust Seismicity studies and studies of moment tensors of earth- system developed along its northern border, where Mesozoic quakes have predicted a similar velocity of both the Aegean rocks overthrust andesitic tuffs as well as Middle Miocene and the Anatolian domains with respect to Europe (Jackson volcanosedimentary units, which themselves are thrust over & McKenzie 1984; Taymaz, Jackson & McKenzie 1991; the Pontus series. The Tosya Basin is similarly located south Jackson, Haines & Holt 1992), suggesting that most of the of the NAF. It is characterized by an asymmetry which shows present strain is released in seismic energy. However, the NAF the major tectonic activity of its northern border during the lateral displacement rate estimated by offsets of geological or Pontus Group sedimentation (Andrieux et al. 1995). The Tosya geomorphic features is about 5-10 mm yr-' (Barka 1992) and and Cerkes-Ilgaz basins were formed during a shortening seems underestimated with respect to the present-day slip rate episode that caused the reverse faulting and folding of the given by the geodesy and seismicity. Miocene volcanosedimentary deposits (Over et al. 1993;
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Figure 2. Simplified tectonic map of the convex-northwards arc of the NAF showing the basins studied (dotted zones), arranged parallel to the NAF Dots show locations of fault-slip measurement sites (identified by numbers, see Table 3, for latitudes and longitudes see Appendix A) C-I B indicates the Cerkes-Ilgaz Basin, T B, the Tosya Basin, K B, the Kargi Basin; H B, the Havza Basin; and L B, the Ladik Basin Years wlth arrows delimit the approximate zones of the reactivated NAF segments during the 1943, 1944 and 1951 earthquakes Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021
Andrieux et al. 1995). In addition, synsedimentary normal kinematics on minor and major faults. The kinematics of a faulting has affected the Lower Pontus in these basins, suggest- fault population can be defined using the striations observed ing a Pliocene episode of extension (Over 1996). Near Kargi on the fault planes. If one assumes that the slip vectors the Pontus series were deformed into a small fold-and-thrust indicated by the striations represent the direction of the belt, while the Pontus series to the north was cut by a high- maximum resolved shear stress on each fault plane (e.g. Bott angle reverse fault. A W-trending active vertical dextral fault 1959), then the observations of slip on numerous fault planes affects the Lower Pontus sediments and is laterally linked to can be inverted to compute a mean best-fitting stress tensor the major segment of the NAF bounding the basin to the (e.g. Carey 1979; Angelier 1984). To determine the stress state southeast. The Havza and Ladik basins are located at the responsible for recent faulting along the NAF, we performed easternmost part of the area studied. The N1 10"-trending such a quantitative inversion of families of slip data, determined major NAF trace follows their northern boundary. The Havza at individual sites, using a method originally proposed by Basin is infilled by Recent lacustrine, fluvial and colluvial Carey (1979), which is one of several existing algorithms (see sediments overlying the Pontus series. Breccias over a 100m references in Mercier, Carey-Gailhardis & Sebrier 1991). This wide zone in Mesozoic marbles mark the major NAF trace in method supposes that rigid block displacements are indepen- this basin. Studies provided evidence that after the initial dent and that the slip vector, s, given by the measured striation deformations of the basins within the central NAF bend, the on each fault plane is parallel to the resolved shear stress, 7 regional strike-slip regime took place within a short late (the tangential stress, i.e. the projection on the fault plane of Miocene-Pliocene (?) overthrusting episode reactivating the applied stress), calculated for each fault plane. This method inherited structures (Andrieux et a/. 1995). This episode was computes a mean best-fitting deviatoric stress tensor from a characterized by NNW- to NW-trending compression, compat- set of striated faults by minimizing the angular deviation ible with a dextral component of the NAF movement. It between a predicted slip vector (z) and the measured striation represents the preliminary stage of deformation along the (s) (Carey & Brunier 1974; Carey 1979). The inversion results central NAF, related to the extrusion of Anatolia, and it was include the orientation (azimuth and plunge) of the principal followed by a Pliocene extensional regime (Over et a/. 1993; stress axes of a mean deviatoric stress tensor as well as a Over 1996). 'stress ratio' R = (02-~1)/(c3-c71),a linear quantity describing The NAF convex bend was the locus of surface ruptures relative stress magnitudes, where [cl[+ [ci21 + Igj[=O. Principal produced by three major historic earthquakes (Fig. 2) stress axes, g1, cr2 and ggrcorrespond to the compressional, (Ambraseys 1970; Barka & Kadinsky-Cade 1988). The intermediate and extensional deviatoric stress axes, 1943 November 26 earthquake (M = 7.6) reactivated the NAF respectively. over 300 km, including segments running from Ismetpasa in the west to Destek in the east of the zone studied. The 1944 February 1 earthquake (M = 7.3) ruptured three segments, Inversion of seismic-slip-vector data sets to determine stress state one located in the western domain of the Ilgaz-Cerkes basin, while the 1951 May 26 earthquake (Ms=6.5) reactivated a To compute the stress states responsible for the present-day 25 km long segment of the NAF, between Ilgaz and Cerkes. faulting (i.e. for focal mechanisms) along the NAF, we per- Numerous historic and Holocene fault scarps around the formed quantitative inversions of focal mechanisms from the convex bend of the NAF are associated with en echelon major earthquakes, using the method proposed by Carey- and discontinuous ruptures, strongly suggesting a strike-slip Gailhardis & Mercier (1987), which is one of several existing component for the latest events. algorithms (e.g. Vasseur, Etchecopar & Philip 1983; Gephart & Forsyth 1984). To compute the stress state from earthquake focal mechanisms requires a knowledge of the seismic slip Methodology: stress determination from fault kinematics vector, and consequently the selection of the preferred seismic fault plane for each pair of nodal planes. The selection can be Inversion fault-slip data sets to determine the stress state of made if there is a coseismic rupture, or by the spatial epicentre Constraints on the Quaternary stress states in the basins distribution of the aftershock sequence. An alternative is to studied and along the NAF are provided by inversions of fault use a computer. Indeed, because only one of the two slip
0 1997 RAS, GJI 131, 61-86 Stress changes along the North Anatoliun Fault Zone 65
vectors of a focal mechanism solution is the seismic fault slip tends towards the same solution regardless of the initial vector in agreement with the principal stress axes, it is possible parameter values (Carey 1979; Carey-Gailhardis & Mercier to compute the seismic fault slip following Bott's (1959) model. 1987; Mercier et al. 1991). Moreover, Bellier & Zoback (1995) For this slip vector, the R ratio, defined as R = (02-cl)/(c3-cl), established specific criteria, which we have used in the current is such that O Reliable inversion results Signijicance of stress-ratio variations As mentioned above, fault-slip inversion schemes are based on The stress ratio, R =(02-ol)/(a3-e1),describes the relative the assumption that the slip direction on each plane represents stress magnitudes of the calculated mean deviatoric stress the direction of the maximum resolved shear stress on that tensor. Indeed, the stress ratio defines the shape of a stress plane. In this case there are four unknowns (three defining the ellipsoid. To compare R-values of distinct stress deviators in orientation of each principal axis and one defining the stress terms of stress-magnitude variation, it is necessary that the ratio, R) and the inversion thus requires at least four indepen- stress deviators compared are coaxial (that is, that they have dent fault sets. Ideal data sets include faults with variable dip the same principal axis directions) in order to be represented angles and distinct strike directions, not just a continuum of on a Mohr's circle. A discussion of the stress-ratio significance strikes around a single mean direction. A slip vector (deter- in interpreting inversion results is presented in Bellier & mined from focal mechanism or by striations, s) is generally Zoback (1995). As defined above, the R-value varies between considered as mechanically explained by a computed stress two endmember uniaxial stress states, that is R=O when 02= deviator when the deviation angle between the calculated slip ol, and R= 1 when 02=03.In a strike-slip-faulting stress vector T and the observed slip vector s is less than 20" (Carey regime (where vertical stress o, = 02, maximum horizontal 1979). Results of stress inversions are considered reliable if 80 stress oHmax= ol, and minimum horizontal stress chmin= c3), per cent of the deviation angles between z and s are less than the R =O endmember corresponds to a stress state transitional 20" and if the computed solution is stable, that is the inversion to normal faulting (extensional regime), in which cHmax=c,, Table 1. Results of regional stress-tensor inversions from the regionally significant focal mechan- isms in northern Turkey, from east to west. They represent a transpressional strike-slip stress state in the eastern domain (RE), a transtensional strike-slip stress state around the central NAF convex bend (RC) and an extensional stress state in the western domain (RW). We divided this last focal mechanism set, RW, into two subsets: a northern subset, RwN, which corresponds to earthquakes close to and within the NAF zone, and a southern subset, RwS, which corresponds to earthquakes close to the Anatolian graben system. N corresponds to the number of focal mechanisms used for the inversions. Deviatoric principal stress axes, u,, u2, u3, are the compressional, intermediate and extensional deviatoric axes, respectively. They are specified by azimuths (Az) measured clockwise from the north; plunges (dips) are measured from horizontal. R =(u2-u1)/(u3-u,) is the 'stress ratio' of the deviatoric stress tensor. The mean deviation angle is M.D.= [Z(z,s)]/N, where (T,s) is the angle between the predicted slip vector, 5, and the observed slip vector, s (i.e. the rake of the focal mechanism). The standard deviation of the deviation angle is S.D.= {[~(T,S)~]/N}~~~.Qty refers to the quality of the stress inversion: + indicates a well-constrained inversion, i.e. when N > 11, M.D. < 13, M.D < S.D < 3/2 M.D, 70" < CL, plunge < 90°, ahminand aHmaxplunges < 20" (as defined in Bellier & Zoback 1995); - indicates an inversion result which did not meet the quality criteria defined above. Region N (TI u2 63 R M.D. S.D. Qy Az/dip Az/dip Az/dip RE-East 10 174"/1" 78"/78" 264"/11" 0.78 I 10.5 - RC-Centre 12 121"/6" 336"/83" 217"/3" 0.37 5.4 6.9 + RwN (Northwest) 6 65"/80" 291"/7" 200"/7" 0.31 RwS (Southwest) 5 129"/80" 286"/9" 16"/4" 0.55 RW-West 11 65"/80" 286"/7" 196"/6" 0.33 3.9 4.9 - 0 1997 RAS, GJI 131, 61-86 66 0. Bellier et al. Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 Compression 01 Intermediate \\\\ Extension (5; Figure 3. (a) Sketch map of the North Anatolian Fault Zone showing focal mechanisms of the major earthquakes (references are given for each earthquake in Table 2). Plots show nodal planes and slip-vector arrows on the preferred seismic fault planes (arrows point in the direction of the horizontal azimuth of the slip vector), the preferred seismic plane being chosen according to the method of Carey-Gailhardis & Mercier ( 1987) (see text). Numbers and letters outside the balloons refer to the focal mechanism labels given in Table 2; Arabic numerals refer to the western region, Roman numerals to the central region, and letters to the eastern region. (b) Stereoplots RW, RC and RE are the lower-hemisphere stereoplots of the earthquake slip data with the present-day regional principal stress directions computed from the focal mechanisms of earthquakes shown in (a) and in Table 2 (results are given in Table 1) for the western, central and eastern regions, respectively, of North Anatolia. Numbers and letters inside the histograms and outside the stereoplots refer to the focal mechanism labels given in Table 2 and outside the balloons in the map in (a). Lower-hemisphere plots show fault planes and measured slip vectors (arrows); arrows point in the direction of the horizontal azimuth 0 1997 RAS, GJI 131, 61-86 Stress changes along the North Aizatolian Fault Zone 67 whereas the R = 1 endmember represents a stress state tran- To determine the regionally significant stress states respon- sitional to thrust faulting (compressional regime), in which sible for modern faulting around the along-strike NAF, we ohmin= o,,. For R-values close to 0 or to 1, the near-transitional used the Carey-Gailhardis & Mercier ( 1987) inversion method (that is near uniaxial when 0.85 of the slip vector. Stress axes obtained from the inversions are shown by diamonds (ul),triangles (u2)and squares (u3). Thick lines on the fault traces give the deviation angle between measured 's' and predicted 't' slip vectors on each fault plane. Histograms show distribution of deviation angles (angle between the observed slip, s, and the predicted slip, t. (c) Sketch map of the Eastern Mediterranean region showing the approximate modern stress field in terms of uHmax(ul or u2)and uhmln(u3) directions, which are related to the Arabian/Eurasian collision and to the Aegean domain (modified after Mercier et a/. 1987; Reba!, Philip & Taboada 1992). Balloons represent the mean stress deviators related to the modern regional stress states that we computed for the western (RW), central (RC) and eastern (RE) North Anatolian regions. 0 1997 RAS, GJI 131, 61-86 68 0. Bellier et a1 Table 2. Parameters of the focal mechanisms from major earthquakes which occurred around the North Anatolian Fault Zone used to determine the eastern, central and western regional stress tensors reported in Table 1. Slip vectors are specified by strike, dip and rake using seismological convention (strike azimuth measured clockwise from north, 90" less than dip direction; dip measured from horizontal, rake gives the direction of motion of the hanging wall with respect to the footwall measured counter-clockwise from the strike direction (positive rakes indicate thrust faulting, negative rakes normal faulting, rakes with absolute values < 90 are left-lateral, rakes with absolute values >90" are right-lateral). In the number column, we show the label numbers and letters associated with the active fault-slip data used to determine the present-day regional states of stress by inversion. The numbers are labels for individual fault planes shown on stereonets and the corresponding histograms RE, RC and RW. The results of these inversions are given as solutions RE, RC and RW in Table 1. M: M, or mb magnitudes are shown.The nodal planes are shown as azimuth and dip for each nodal plane (Planes 1 and Planes 2), while T axes are given as azimuth and plunge. In the Reference column the references are as follows. 1: Jackson & McKenzie (1984); 2: McKenzie (1972); 3: Dziewonski, Franzen & Woodhouse (1984a,b), Dziewonski et al. (1989, 1991) and Dziewonski, Ekstrom & Salganik (1993); 4 Stewart & Kanamori (1982); 5: Ritsema (1974); 6: Contantinescu, Ruprectova & Enescu (1966); 7: Wickens Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 & Hodgson (1967); 8: Canitez & UGer (1967); 9: McKenzie (1978). N Date Lat" Lon' Planes1 Planes 2 T-axis M Ref strike"/dip" strike"/dip" Az/pl (a) Eastern part A Dec. 26 1939 39.5 38.5 200/61NW 108/86SW 60123 7.9 2 B July 26 1967 39.5 40.4 194172NW 102184SW 6011 1 5.9 2 C Mar. 13 1992 39.9 39.6 21 3/85NW 123186SW 7817 6.8 3 D Apr. 20 1990 39.9 39.7 299190 29/90 2 5410 6.9 3 E July 7 1957 39.2 40.3 23815 1NW 11715 1 s w 81/63 5.5 8 F Aug. 20 1966 39.4 40.9 104186SW 13/76SE 240110 5.3 2 G Aug.19 1966 39.2 41.6 50/61 SE 304/64NE 266141 7.1 2 H Mar. 7 1967 39.1 41.6 60156SE 3 10160NE 4610 1 5.6 2 I May 22 1971 38.8 40.5 143/82SW 52186SE 98/01 6.9 1 J Sept. 6 1975 39.5 40.7 114/5OSW 244/54NW 91/62 6.7 1 (b) Centiral part I June 20 1943 1.0 30.0 266190 176176SW 40110 6.2 2 I1 July 30 1967 40.7 30.4 301/50SW 151144sw l08/82 5.6 2 111 May 26 1957 40.7 31.9 178/90 0871788 43/12 7.1 2 IV May 27 1957 40.5 30.0 28 1j88NE 192/67NW 234118 6.2 5 V July 22 1967 40.7 30.8 183190 093190 228101 6(m!J 2 VI Febr. 20 195640.0 32.5 264150NW 140/56SW 96/55 6 2 VII Sept. 27 1953 41.2 33.8 280/75NE 187/70NW 51/02 6.2 8 VIII May 10 1977 41.0 33.4 348/88NE 258/80NW 220108 5.8 1 IX Dec. 10 1966 41.0 33.5 255190 345190 230100 4.9 2 X Aug. 13 1951 40.8 33.4 348183NE 81170SE 3711 1 6.7 2 XI Nov. 26 1943 41.0 33.4 001/83E 269/73N 226117 7.6 7 XI1 Dec. 20 1942 40.5 36.5 345157NE 228156NW 197151 7.3 7 (c) Western part 1 Mar. 27 1975 40.4 26.1 041/60SE 279146NE 157107 5.7(m,) 9 2 Sept. 22 1939 39.0 27.0 10015 1sw 250/43NW 176105 6.5 5 3 July 5 1983 40.3 27.2 149166SW 248170NW 18/02 5.6(mtJ 3 4 Mar. 18 195340.0 27.5 57/80SE 324/70NW 189106 7.2 6 5 Oct. 6 1964 40.3 28.2 287154NE 115/36SW 21/09 6(md 4 6 Oct. 24 1988 40.8 28.7 3 5617 1E 89/80S 283108 6 3 7 Sept. 18 1963 40.9 29.2 118/2OSW 276170N 12/26 6.4 2 8 Mar. 28 1969 38.6 28.5 101/61SW 281/29NE 191116 6.4 2 9 Apr. 19 1970 39.1 29.8 284166NE 104124sw 15/23 5.4 9 10 Mar. 28 1970 39.2 29.5 130163SW 260139NW 200114 7.3 5 11 Apr. 16 1970 39.0 30.0 277/55NE 101/35SW 10108 5.5 9 allows one to compute a regionally significant stress deviator Conclusions on the modern stress states around the NAF combining the data sets from the northern and southern regions. This constrained inversion from the combined data Present-day strike-slip to normal-faulting stress regimes along (Fig. 3 and Table 1) yields a result that confirms the local the NAF have been determined by inversions of earthquake individual inversions. It indicates a N16"E-trending (r3 exten- slip vectors deduced from sets of constrained focal mechanisms. sional stress state with an R-value of 0.33, which indicates a The present-day stress field is characterized by transpression triaxial tensor. in the east, changing to transtension in the centre and then to 0 1997 RAS, GJ1 131, 61-86 Stress changes aloiig the North Anutoliaiz Fault Zone 69 extension towards the west (Fig. 3). This explains the coexist- ence of strike-slip and normal-faulting earthquakes along the NAF. In the next section, we will show that the present-day stress state determined by inversion of the slip recorded by 40.2 earthquakes agrees with the stress deviator computed from the @233 youngest striae measured on fault planes, and that the strike- slip stress regime changed during the Quaternary in the central NAF zone. FAULT KINEMATICS ANALYSIS FOR THE QUATERNARY STRESS REGIMES ALONG THECENTRALNAF As mentioned previously, the aim of this paper is to characterize the Quaternary to present-day stress states along the Central NAF. Unfortunately, it is generally difficult to date the stri- Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 ations more precisely than being younger than the age of the rocks cut by the faults. In this study we included results of measurements made in Mesozoic bedrock and Cenozoic basin sediments, that is the Plio-Pleistocene Pontus Group and Recent colluvial and alluvial deposits. Often, more than one Figure 4. Chronology of two families of slip directions measured on set of striae are present on a fault plane at the same measure- faults at several sites. Fault planes and measured striation directions ment locality. The separation of distinct families of striations (arrows) are shown in a lower-hemisphere stereographic projection; must be carried out on the basis of geological field relationships, arrows point in the direction of the horizontal slip azimuth. Labels including relative chronology of the striations (cross-cutting outside and to the right of the stereonets refer to sites located in Fig. 2 relationships) and their relation to regional tectonic events. and listed in Appendix A. Numbers describe individual fault-plane This methodology of fault kinematic studies in determining measurements; the older slip vector on each fault plane is indicated palaeostress fields and demonstrating temporal changes in by (.l),the younger by (2). stress states has been developed by previous authors (see Mercier et al. 1979, 1987, 1989, 1991; Angelier et al. 1982; plotted in Figs 5 and 6 for the transpressional and transten- Sebrier et al. 1988; Bellier et al. 1991; Bellier & Zoback 1995). sional regimes, respectively. Inversion results for all sites are At several localities (the locations of sites of fault-slip given in Tables 3 and 4. Below, we report results of kinematic measurements are given in Fig. 2 and Appendix A), reverse- analysis of the Quaternary stress regimes for some significant component strike-slip striations were found to be cross-cut by localities. normal-component shallow-rake strike-slip striations on the The first example is site 10, which is located along the same fault plane. These two contrasting sets of striae are Gerede Cay valley, 15 km NNE of Cerkes. At this locality, the observed along major fault planes or adjacent faults affecting NAF has recent scarps at its base and right-lateral stream rocks ranging in age from Mesozoic to Plio-Pleistocene, that offsets (Fig. 7). This faulting occurs mainly at or near is they affect rocks from the basement up to the Pontus Group. the alluvium/bedrock-range or alluvium/volcanic-range front We show on lower-hemisphere stereoplots examples of slip boundary. An exposure of the normal strike-slip fault (Fig. 7c) chronologies measured on several independent fault planes confirms the recent normal-component faulting of the central (Fig. 4). The two families of cross-cutting striae, shown by NAF. Slip measurements at site 10, located 1 km south of the differences in rake angle of about 35 f 10" measured at several major fault trace, were made on striations on fault planes that sites along the NAF, suggest a change in the NAF strike-slip affect Palaeogene volcanism. This striae set records two con- stress regime. Kinematics from the youngest striae affecting trasting strike-slip-faulting subsets: the first with a reverse the Mesozoic and early Tertiary rocks generally agree with the component and the second with a normal component (stereo- fault slips from the striations affecting the Plio-Quater- plots 10 in Figs 5 and 6), which seem to represent two distinct nary formations. This younger rake value agrees with the stress regimes. As few normal-component striations were dominantly normal component of the strike-slip faulting of collected, they could not be inverted. However, the reverse- earthquakes around the central NAF (Fig. 3). component data set permitted an inversion which yielded a In summary, the data reported in this study provide evidence strike-slip regime with a N124"E-trending o1 and an R-value of strike-slip faulting affecting Mesozoic bedrock as well as of 0.71 (Table 3). Recent formations, and indicate a change in the strike-slip The second significant example is site 33, located 10 km regime from reverse-component to normal-component faulting. northeast of Ilgaz. There are faults in the bedrock, exposed by Thus, this change probably marked a transition between a road-cut between IIgaz and Kastamonu. This site, to the transpressional (reverse-component) and transtensional northeast of the Ilgaz Basin border, is located just south of (normal-component) strike-slip regimes. Lower-hemisphere the major ENE-trending NAF fault zone, marked by an stereoplots of all fault-kinematic and inversion results (for sites approximately 14&150 m high fresh-looking escarpment. Site with sufficient data to calculate stress deviators), and histo- 33 provides Mesozoic marble slickensides, and striations (well- grams of deviation angles [the angle between the striae (s) and preserved grooves and frictional striations) were measured on the predicted slip (z) directions)J and principal deviatoric metre-scale fault planes, which are secondary fractures within stresses (ol,o2 and 03),as well as stress ratio values, R, are the fault zone. On several fault planes the older striae set 0 1997 RAS, GJI 131, 61-86 30 5&6 R 0.865 R 0.827 01 145. 3. -1.02 01 149. 0. -0.99 02 244. 71. a2 19. 90. 0.36 03 54. 18. a3 239. 0. 0.64 i 10 20 30 (T. 10 20 30 40 (T, ’) 10 33 R 0.713 R 0.787 a1 124. 5. -1.02 a1 117. 4. -0.95 Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 a2 228. 80. 0.31 N a2 346. 83. 0.25 N a3 214. 5. 0.77 a3 26. 9 10 20 30 40 (T,’) 10 20 30 (T. ’) 34-37 11&12 R 0.798 R 0.516 01 310. 10. -1.08 a1 144. 19. -0.96 02 169. 77. 0.36 N a2 345. 70. 0.02 N 03 41. 8. 0.72 a3 236. 7. kB, -3 10 (7. S) 19-21 44 R 0.663 R 0.936 01 138. 0. -1.07 a1 314. 9. -1.06 02 324. 90. 0.21 02 197. 70. 0.48 a3 228. 0. 0.86 A a3 47. 17. 0.58 Figure 5. Lower-hemisphere stereoplots showing reverse-strike-slip faulting and local reverse-slip faulting data from the Central NAF region, together with results determined by Carey’s (1979) inversion method, as given in Table 3. Labels outside and to the right of the stereonets refer to sites located in Fig. 2 and listed in Appendix A. The results include deviatoric stress parameters (al,u2 and a3 axes) determined by Carey’s (1979) inversion method, that is azimuth, plunge and relative magnitudes of the principal axes as well as the stress-ratio value (R= u2-a,/a3-a1).See caption to Fig. 3(b). 0 1997 RAS, GJI 131, 61-86 Stress changes along the North Anatolian Fault Zone 71 46-49 618~62 R 0.854 R 0.779 a1 166. 3. -0.05 01 143. 5. -1.05 a2 280. 82. a2 251. 73. 0.33 ti a3 76. 7. a3 51. 16 3 T Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 548L55 67 R 0.961 R 0.617 01 156. 0. -0.92 a1 326. 6. -0.99 02 a2 201. 80. 0.14 60. 89. a3 246. 1. TI 10 20 30 110 (.. ') 68k69 56&57&60 R 0.650 R 0.888 a1 147. 1. -0.99 a1 149. 11. -1.01 a2 251. 84. 0.18 N a2 270. 69. 0.41 03 55. 18. 0.59 T Ifiq 10 20 30 40 jT. s, 10 20 30 58&59 R 0.837 01 140. 1. -1.06 02 43. 78. 0.39 n a3 230. 12. 10 20 30 (T. s, Figure 5. (Continued.) 0 1997 RAS, GJI 131, 61-86 72 0. Bellier et al. 9 11&12 R 0.661 R 0.577 a1 9. 16. -0.84 01 154. I. -0.79 a2 101. 8. a2 244. 8. 0.08 03 216. 72. 0.67 a3 60. 82. 0.71 # 36Y 10 20 30 '40 50 s, Figure 5. (Continued.) Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 records a slightly reverse-component strike-slip set overprinted that indicate two distinct strike-slip faulting types, normal- by younger normal-component lateral-slip (site 33 on Fig. 4). and reverse-component. To constrain the inversions for com- Because of the evidence for this chronology, we divided the puting the corresponding stress deviators, we combined data data into dominantly normal-component and dominantly from sites 54 and 55 (54855 in Tables 3 and 4, stereonets reverse-component strike-slip-faulting subsets, representing 548L.55 in Figs 5 and 6). Data-set inversions yielded results two distinct strike-slip regimes (stereoplots 33 in Figs 5 and which consistently confirm two distinct strike-slip stress states, 6). As indicated in Tables 3 and 4 and on the stereoplots, the both characterized by NW-trending o1 axes (N34"W in Table 3 inversions of these data sets yielded two strike-slip-faulting and N27"W in Table4) and NE-trending a3 (N57"E and stress regimes with consistent WNW-trending o1 axes for both N64"E), the calculated R-values being 0.62 and 0.29, events, i.e. N117"E, and stress-ratio values R of 0.78 and 0.39, respectively. respectively Faults with normal vertical offsets (of decimetres to metres) Another significant example is site 54, located 30 km east of affecting Pliocene lacustrine units and Quaternary deposits the city of Tosya, where an outcrop showing the Upper Pontusl that fill the basins studied are also observed in several places. bedrock faulted contact is exposed in the easternmost Tosya An example of these faults is exposed in a clay quarry excavated Basin. The faulting occurs at the contact of the Upper Pontus in the Ilgaz Basin along the Kursunlu-Ilgaz road (site 27, conglomerates and the Mesozoic marble (Fig. 8). This fault Fig. 2). Normal faulting at this locality offsets Lower Pontus outcrop shows dragged beds along the fault plane and a fan- Formation Pliocene sedimentary deposits, which consist of a like pattern for the Upper Pontus conglomerate dip, indicating poorly consolidated silt and clay unit (Fig. 9). In the majority the synsedimentary character of part of the exposed faulting. of cases, the measured fault planes strike in several directions The outcrop also shows a recent poorly consolidated silt, sand and are overprinted by oblique-normal-slip and strike-slip and gravel unit which overlies the deformed Pontus Formation striations. The cross-cutting striations show a chronological with a slight unconformity, the fault zone being sealed by relationship between the older strike-slip faulting (1 on Fig. 9) recent silty microconglomerates. A Pleistocene age for the and younger normal faulting (2 on Fig. 9). The normal-faulting Upper Pontus Formation is likely in the eastern Tosya Basin, striae agree roughly with NE-trending extension, while the because the deformed formation described here is situated just strike-slip striations indicate NNW-oriented compression and above the Pliocene lacustrine Lower Pontus Formation (Over ENE-oriented extension. Near to site 27, another excavation 1996). Thus, this faulting occurred during Pleistocene Upper in the Lower Pontus Formation exposes strike-slip faults 2 km Pontus sedimentation and before the Holocene. Slip data are south of the city of Ilgaz (site 28, Fig. 2). Similarly, this faulting mechanical striations collected along the fault plane and on affects clay sediments. To constrain the strike-slip data inver- secondary fractures within the fault zone affecting marbles. sion, we combined data from sites 27 and 28 (278~28in Table 4 Even if the faulting affects Pleistocene Upper Pontus sediments, and stereonet 278~28 on Fig. 6). The inversion from this striations are only observed in the marble. At this locality the combined data set yields a well-constrained strike-slip stress major fault planes strike WNW and are marked by slickensides state characterized by a N24"W-trending g1 and a N66"E- overprinted by thin strike-slip frictional striations which agree trending o3 and an R-value of 0.42, while the normal-striation roughly with NW-trending compression. The fault-slip data inversion yields a consistent result indicating an extensional set clearly records two distinct strike-slip faulting events, the stress regime (a, vertical) with a N42"E-trending o3 and an first with a dominantly reverse component and the second R-value of 0.60, as shown in Fig. 6 (stereonet 27). with a normal component, these two slip-vector sets corre- Other brittle deformation in the basins studied provides sponding to two stress states. The synsedimentary deformation evidence for recent normal faulting (sites 24, 52, 53 and 54 in observed in the outcrop shows a reverse drag fold, providing Fig. 2 and Table 4b). For instance, at site 24, in the Ilgaz Basin, further evidence that reverse-component faulting affects the normal faults with offsets of several decimetres affect Recent Upper Pontus (Fig. 8). Another example of such a strike-slip sediments and vertically displace the topography (Over et al. fault is exposed in a quarry south of the Tosya-Havza road, 1993). All these normal-faulting sites are generally characterized at site 55 (Fig. 2) east of the Tosya Basin, 4 km east of site 54. by silty-clay slickensides overprinted by few frictional oblique- It shows similarly faulted Mesozoic marbles with striations slip striations. As only a few striations were collected, they 0 1997 RAS, GJI 131, 61-86 Stress changes along the North Anatolian Fault Zone 73 Table 3. Results of stress-tensor inversion for slip data representing transpressional strike-slip faulting stress regimes which pre-date the present-day stage. In the Site column, '&' indicates an inversion solution computed from two or more data sets from different sites; for example, 1&2&3 corresponds to an inversion computed from the data sets of sites 1, 2 and 3. '-' indicates an inversion solution computed from more than two data sets from different close sites; for example, 46-49 corresponds to an inversion computed from small data sets from the close sites 46, 47, 48 and 49. N = number of striated fault planes used to compute the solutions. Individual columns are the same as in Table 1. Reference ages are reported in Appendix A. (a) Results of stress-tensor inversions for slip data representing strike-slip faulting stress regimes. 'SS.l' is an average regional stress state obtained from computing mean stress axes using Fisher statistics on individual horizontal 5, and u3 axes. R, value is the arithmetic mean of all sites. (b) Results of local stress- tensor inversions for slip data representing local thrust-faulting stress regimes. Sites N 51 52 53 R M.D. S.D. Qy Age Azjdip Azldip Azldip (a) 52x6 20 145"/3" 244"/71" 54"/18" 0.86 14.0 15.5 - Pa Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 10 17 124'15" 346"/83" 214'15" 0.71 12.1 14.0 + Pa 11&12 13 144"/19" 345"/70" 236"j7" 0.52 09.3 10.9 + PI 19-21 17 328"/11" 95"/72" 235"/14" 0.52 14.1 16.5 - M 30 10 145"/13" 28"/63" 241"/23" 0.85 09.2 10.6 - Jr-Cr 33 25 11714" 231"/80" 26"/9" 0.78 11.7 15.2 + Jr-Cr 34-37 5 3 10"/10" 169"/77" 41"/8" 0.80 06.6 08.7 - M 44 15 314"/9" 197170" 47"/17" 0.94 08.3 09.5 + Jr-Cr 46-49 7 166"/3" 280"/82" 76"/7" 0.85 10.5 13.8 - P-PI 54&55 26 326"/6" 201"/80" 57"/8" 0.62 15.7 19.1 - Jr-Cr + PI 56&57&60 9 149"/11" 270"/69" 55"/18" 0.89 17.1 19.7 - Jr-Cr + P 58&59 12 140"/1" 43"/78" 2 3Oo/l2" 0.84 14.8 18.0 - Jr-Cr + Pa 61&62 29 143"/5" 251"/73" 51"/16" 0.78 12.5 16.2 + Jr-Cr 67 21 158"/2" 65"/59" 249"/31" 0.90 07.9 10.3 + Jr-Cr 68&69 27 147"/1" 251"/84" 57"/6" 0.65 12.8 16.6 + Jr-Cr ss1 ul: 142+ 8"/3" cr3: 52*13"/1" R,: 0.75 ( b) 9 12 9"/16" 101"/8" 216"/72" 0.66 14.0 19.2 - PI 11&12 16 154"/1" 244"/8" 60"/82" 0.58 10.6 12.8 + PI Table 4. Results of stress-tensor inversion for slip data representing transtensional strike-slip- faulting stress regimes. Individual columns are the same as in Table 1. (a) Results of stress-tensor inversions for slip data representing strike-slip-faulting stress regimes. 'SS.2' is an average regional stress state obtained from computing mean stress axes using Fisher statistics on individual horizontal c1 and u3 axes. 'SS.fm' is the regional stress state for the same tectonic regime obtained from focal mechanism inversion (see RC in Table 1). (b) Results of local stress-tensor inversions for slip data representing local normal-faulting stress regimes. Sites N c1 52 53 R M.D. S.D. QY Age Azldip Az/dip Az/dip (a) 1&2 17 122"/2" 2"/86" 213"/3" 0.09 14.4 17.2 - Cr 3&4 33 322"/5" 111"/84" 232"/3" 0.06 12.6 14.3 + Cr 10 4 NW-SE NE-SW Pa 13 9 328"/22" 168"/66" 61"/7" 0.13 08.2 11.2 - Cr 27&28 35 336"/3" 124"/86" 246"/2" 0.42 08.6 10.7 + P-PI 33 14 117"/0" 285"/90" 27"/0" 0.39 07.4 10.0 + Jr-Cr 43 13 328"/15" 119"/73" 236"/8" 0.42 11.7 13.9 + Jr-Cr 54&55 16 153"/9" 13"/78" 244"/8" 0.29 09.9 13.1 + Jr-Cr + PI 61&62 34 324"/5" 186"/83" 54"/5" 0.17 11.2 14.2 + Jr-Cr ss.2 ol:322k 14"/5" u3: 232 10"/2" R,: 0.24 SS.fm ul: 127"/6" 03:217"/3" R: 0.39 ( b) 24 3 NE-SW Pl+H 27 12 328"/72" 130"/18" 225"/5" 0.60 05.8 07.0 + P-PI 52-54 7 141"/83" 323"/7" 233"/0" 0.64 05.1 05.9 - Jr-Cr + PI 1997 RAS, GJI 131, 61-86 74 0. Bellier et al. 33 R 0.393 01 117. 0. -0.79 02 2. 86. 02 285. 90. -0.12 N 03 213. 3. 03 27. 0. 1 7 Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 3&4 43 R 0.064 R 0.416 01 322. 5. -0.57 01 328. 15. -0.88 02 111.84. -0.47 *I 02 119. 73. 03 236. 8. 4 10 20 30 (T. ’) 54&55 13 R 0.290 R 0.129 a1 153. 9. -0.71 01 328. 22. -0.59 02 13. 78. 02 168. 66. -0.39 w 03 244. 8. u3 61. 7. 0.98 10 20 30 (T. ’) 27&28& 16 616162 R 0.416 R 0.174 01 336. 3. -0.78 a1 324. 5. -0.67 02 186.~~ 83.~ -0.37 03 54. 5. Figure 6. Lower-hemisphere stereoplots showing normal-strike-slip and local normal-slip faulting data from the Central NAF region, together with the inversion results presented in Table 4. See Figs 3(b) and 6. 0 1997 RAS, GJI 131, 61-86 Stress chunges along the North Anutoliun Fault Zone 15 10 27 R 0.597 01 328. 72. -0.82 02 130. 18. 0.10 03 222. 5 T a:ll2 29 6 Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 52-54 R 0.642 24 01 141. 83. -0.79 02 323. 7. 0.14 Y 03 233. 0. 0.66 h,2, 22 10 IT. S) Figure 6. (Continued.) cannot generally be inverted, but they agree with approximately around N35-50"W), a NE-trending Ohmin (u3 axes trending NE-trending extension. Inversion of combined faults from sites between N26"E and N76"E with a maximum around 52-54 yields an extensional stress regime with a N53"E-trending N40-WE) and R-values ranging between 0.52 and 0.94 up This extensional stress state may be interpreted as a local (Fig. 10). The u3 axes and R-values for the high-quality inver- effect of the regional transtensional regime. sions (seven out of 15 inversions, Table 3) covered the entire range. The calculated mean deviator of the Fisher statistic yields a regionally significant stress state characterized by a ul Regionally significant transpressional and transtensional axis trending N142 f8"E and a u3 axis trending N52 f 13"E, strike-slip regimes both axes having a plunge of 2" (SS.l in Table 3a and Fig. 10; In an attempt to constrain the parameters of the strike-slip note that the quoted azimuths of 8" and 13" correspond to the stress regimes at a regional scale, we computed mean stress radius of the 95 per cent cone of confidence in Fisher's axes with their 95 per cent confidence ellipses using Fisher statistics). The mean R-value of 0.75 (Rm in Table 3a) indicates statistics (as modified by Watson 1960). This statistical analysis that the regional stress regime is transpressional. Most of the was carried out independently for each subset of individual R-values are close to 1 (a majority of R-values are between 0.7 site subhorizontal stress axes (SS.l and SS.2 in Tables 3 and 4 and 0.9), as shown by the histograms of R-value distribution and Fig. 10). (Fig. 10). Three of the seven well-constrained, high-quality inversions yield R-values between 0.7 and 0.8. This means that the Ohmin axis is close in magnitude to a,, indicating a regional The reverse-component strike-slip stress regime strike-slip regime close to the transition to a reverse-faulting regime. This is consistent with the local reverse faulting Evidence for the reverse-component strike-slip stress regime is observed in some places (sites 9 and 118~12in Table 3b and recorded throughout the central NAF domain by slip on faults Fig. 1la). In these localities, brittle deformation shows reverse which affect Pliocene and early Pleistocene fluvio-lacustrine oblique-slip striations that agree with an approximately NNE- deposits, as well as Tertiary volcanism and Mesozoic bedrock. to N-trending compression, for which the inversions yield Slip data were measured both along major fault planes and reverse-faulting stress states (uV=u3). within adjacent minor faults along fault zones and in the basins. Individual site u3 and u1 directions (given in Table 3) The normal-component strike-slip stress regime are reported on the map in Fig. ll(a). All stress deviators indicate a strike-slip stress regime with a NW-trending crHmax Striae measured at several sites along and around the NAF (ulaxes trending between N14"W and N64"W with a maximum indicate a young normal-component strike-slip deformation 0 1997 RAS, GI1 131, 61-86 76 0. Bellier et al. Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 ENE /’ ’//’ / 0 1997 RAS, GJI 131, 61-86 Stress changes along the North Anatolian Fault Zone 77 Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 Figure 8. (a) East-facing view of a fault contact between early Pleistocene Upper Pontus deposits and Mesozoic marble exposed in a secondary road-cut in the eastern Tosya Basin. (b) Interpretative cross-sectional sketch of this apparent reverse strike-slip fault exposed at site 54. Figure 7. (a) South-facing view of the NAF fault scarp along the Gerede Cay Valley close to the village of Ulukoy, 15 km NNE of Cerkes (site 10 on Fig. 2). (b) Sketch of (a), with a box indicating the area shown in (c) and a black line and arrows following the approximate trace of the active fault. (c) East-facing view of an NAF contact fault exposure, with white arrows indicating the top and base of the fault plane. 0 1997 RAS, GJI 131, 61-86 78 0. Bellier et al. Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 Figure9. North-facing view of faulting exposed by a quarry in the Pliocene Lower Pontus Formation at site 27 in the Ilgaz Basin. This fault plane shows examples of cross-cutting striae families between older strike-slip-faulting striae, indicated by the number 1, and younger normal- faulting striae, indicated by the number 2. stage which post-dates the transpressional deformation. This Table 4(a), inversions of these data consistently indicate a stage is recorded throughout the central NAF domain by slip strike-slip stress regime with c1 trending between N22"W and on minor and major faults which affect Neogene to Quaternary N64"W, and c3 between N27"E and N66"E. Six of the height fluvial, lacustrine and colluvial formations, as well as Cenozoic inversions are well constrained and yield similar results to the volcanism and Mesozoic bedrock. As shown in Fig. ll(b) and other ones. To determine the parameters of this stress state at Figure 10. (a) Quaternary regional stress states in the central NAF bend. The transpressional (SS.1) and transtensional (SS.2) lower-hemisphere summary stereoplots show local 'horizontal' stress axes determined from fault striation measurements at each individual site. SS.1 shows results for the transpressional strike-slip-faulting slip inversions given in Table 3(a), while SS.2 gives the results for the transtensional strike-slip-faulting inversions given in Table 4(a). Stars and triangles refer to the u1 and u3 axes, respectively, given by inversion (Tables 3a and 4a). The ul, u2 and u3 axes of the mean regional horizontal stress state, determined using the Fisher statistic method (modified by Watson 1960) applied to each of the local horizontal stress axes, are shown as stars and triangles in circles. The dotted areas correspond to 95 per cent confidence cones for the mean directions. (b) Histograms show the distribution of computed stress-ratio R-values for each individual inversion. Numbers inside the histograms refer to field site numbers given in Tables 3(a) and 4(a), which correspond to labels outside the individual stereoplots in Figs 5 and 6 and to labels in Appendix A. Dotted squares in histograms show R-values for the well-constrained inversions (pluses in Tables 3a and 4a). (c) and (d) Mohr's circle representations of the coaxial stress states for the two regional transpressional and transtensional regimes showing the relative magnitudes of the principal stress axes deduced from the mean R-value (R,,,). (c) represents the shape of the deviator ellipsoids while (d) represents the theoretical stress state. 0 1997 RAS, GJI 131, 61-86 Stress changes along the North Anatolian Fuult Zone 79 ss.l ss.2 Transpression (R>0.5) Transtension (Rc0.5) N N Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 Shear Deviatoric Deviatoric stress Rm = 0.75 Shear Rm = 0.24 (I ui Normal ?I 2 “I Normal stress stress (d) Shear stress R close to 1 I R close to 0 stress 0 1997 RAS, GJI 131, 61-86 1 - REGIONAL TRANSPRESSIONAL STRIKE-SLIPSTRESS STATE 000 P 2 - REGIONAL TRANSTENSIONAL STRIKE-SLIPSTRESS STATE I 2' - LOCAL EXTENSIONALSTRESS STATE I I 35- 4 0' 4 +Oc= e0 p,3 W to3 4W Transpression (0,5 Figure 11. Simplified tectonic maps of the convex-northwards arc of the NAF and the basins studied. Azimuths of cr3(extensional stress)and crl (compressional stress) axes for the Quaternary stress regimes are shown. Dots show approximate locations of fault-slip measurement sites (identified by numbers); exact site locations are reported in terms of their latitudes and longitudes in Appendix A. (a) refers to the Plio-Pleistocene transpressional strike-slip stress regime deduced from the fault slip given in Table 3. Black arrows indicate the crl azimuth and white arrows indicate the cr3 azimuth of these transpressional stress states (Table 3a). At sites 9 and 46-49. we only report cr,azimuths because the corresponding inversions determined local reverse-faulting stress states with vertical crl (Table 3b). (b)refers to the Pleistocene present-day transtensional strike-slip stress regime deduced from fault-slip data given in Table 4(a). White arrows indicate the 6, azimuth while black arrows indicate the cr3azimuth of these transtensional stress states. Fin corresponds to the crl and miazimuths deduced from the inversion of the focal mechanisms. (c) refers to recent local extensional stress states given in Table 4(b) and resulting from the transtensional regime. Black arrows indicate the cr3azimuths of these extensional stress states. Dashed arrows are directions deduced by graphical analyses, when inversion could not be constrained. Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 October 01 on guest by https://academic.oup.com/gji/article/131/1/61/752552 from Downloaded Stress changes along the North Anatolian Fuult Zone 81 the regional scale, we calculated the Fisher-statistic mean stresses (Fig. 2). Along the NAF, the stress regime is NNW- deviator. It is characterized by a N38- 14'W (N322' f 14"E trending o1 transpression in eastern Anatolia, which progress- with a plunge of 5") o1 axis and a N52'i 10'E (N232"+ 10"E ively changes westwards to a NW-trending ol transtensional with a plunge of 2") (r3 axis (SS.2 in Table 4a and Fig. 10). The regime around the central NAF. This lateral variation means mean arithmetic R, value of 0.24 (Table 4a) and the R-value a relative decrease of the oHmaxmagnitude and evolves west- distribution reported by the histograms in Fig. 10 confirm the wards to an extensional regime characterized by a NNE- transtensional character of this youngest stress regime. This trending 03.The oHmaxmagnitude variation induces a change geologically determined strike-slip stress state is remarkably of the vertical principal stress axis, ov,from o2 to ol,leading consistent with the present-day stress regime deduced from the to a change of the stress axes. Thus, the significant change in inversion of earthquake slip, which indicated subhorizontal the regional stress state, compressional versus extensional, in N37"E- and N53"W-trending o3 and ol,respectively. It is North Anatolia seems to have occurred around the central clearly distinct from the transpressional stress state and pro- NAF bend. Present-day spatial variation of the stress field duces locally normal minor faults in the NAF bend. This along the NAF results from the coeval influence of the eastern normal faulting locally affects Holocene formations and slightly and western boundary forces, that is NNE-trending extension offsets the topography. It results from NE-trending extension consistent with Aegean tectonics in the west, and NNE- (Fig. llb and Table 4b). This direction is consistent with the trending compression due to the northward drift of Arabia in Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 u3 direction of the regionally significant transtensional regime. the east. The shear forces along the NAF related to the Analysis of large-scale fault scarps and offsets along the accommodation of the Anatolian extrusion are superimposed central NAF bend indicates a normal component for the recent over these boundary forces. These superimposed forces produce strike-slip faulting and thus confirms the fault-slip-vector study. oHmaxand ohminvalues of the transtensional regime acting in Indeed, despite morphological evidence of strike-slip faulting, north-central Anatolia that are intermediate in direction and such as ridges and en kchelon scarps, we also observed a normal component for the Recent faulting. Blocks bounded magnitude between the oHmaxand ohmin values of the exten- by the active NAF trace show fresh scarps at their bases, sional and transpressional regimes acting at both extremities where stream offsets are exposed along the fault trace, as for of Anatolia. Thus, the central NAF stress state is intermediate example the fault scarp along the Gerede Cay valley close to between the stress states acting in eastern and western Anatolia. Ulukoy village (Fig. 7, site 10 on Fig. 2) or at Koprubasi in As mentioned above, the change in the regional stress the Istavlos Cay valley (Fig. 12). This fault trace, which is pattern seems significant around the central NAF bend. located 15 km from Havza in the Havza Basin, presents Recent However, the present-day stress regime in this area is complex. fault scarps laterally arranged in an 'en echelon' pattern, cutting Indeed, the regime along the NAF and the far-field regime in Recent alluvial deposits. A series of small fresh triangular facets north-central Anatolia are characteristically transtensional, in the footwall block along portions of the fault zone indicate while the regime in the Eurasian central Pontide domain just the normal component of the strike-slip movement. This is northeast of the NAF bend is characterized by transpression, also emphasized by some recent pediment surfaces in elevated which maintains a high topography (Andrieux et al. 1995). positions. Many faults along the active NAF are range- This spatial stress change may result from the combined effects bounding fault zones marking asymmetric active deep valleys, of the southwestward increase of the Anatolia mass transfer for example the 140-150 m high fresh-looking escarpment and the peculiar geometry of the NAF bend (Fig. 13). Indeed, which represents the fault trace along the southern steep slope this increase induces stretching deformation in the Anatolian of the deep Gokqay Valley to the NE of the Ilgaz Basin border. block, that is traction within the Anatolia south of the NAF Some of these faults are associated with minor surface ruptures bend, while the Pontide zone NE of the NAF could be propagating into Recent alluvial basins. At Ismetpasa, where maintained under compression. strike-slip creep is offsetting a wall of the Ismetpasa station, the major Quaternary fault scarp runs less than 1 km to the south. This main fault zone consists of fresh-looking scarps Temporal stress state change in the Quaternary tectonic underlain by a 60-70cm high scarplet dipping at 40" which regime in the central NAF marks the trace of a normal-component strike-slip fault. Thus, In the central NAF bend, both the youngest striae and the large vertical relief, the triangular facets and the vertical earthquake slips indicate Quaternary to present-day domi- offset of apparent Quaternary surfaces imply a normal com- nantly strike-slip stress regimes which induce strike-slip fault- ponent for the recent strike-slip faulting acting in the central ing, affecting Mesozoic bedrock to Recent formations and NAF zone. producing new faults or reactivating older ones. However, In conclusion, the youngest slip recorded by brittle defor- major (at the fault-scarp scale) and minor (at the slickenside mation, seismic slip deduced from the focal mechanisms of scale) deformation provides evidence for a recent change in major earthquakes and large-scale fault-scarp analysis lead the stress state within the strike-slip regime in this region. This to the same conclusion: the regionally significant Recent to present-day stress regime is transtensional. change, shown by striae chronologies and fault-slip-vector inversions, marks the limit between two successive strike-slip- faulting phases, the first having a reverse component and the CONCLUSION: TEMPORAL AND SPATIAL second a normal component. It corresponds to a change STRESS-REGIME CHANGES ALONG THE from transpression to transtension, with consistent NE- and NAF NW-trending o3 and o1 axes. Both the older transpressional and the younger transtensional deformation affected the early Spatial change of the modern stress regime along the NAF Pleistocene Upper Pontus Formation. Only the younger The present-day stress pattern along the NAF is characterized deformation affected Holocene deposits. The transpression was by a change in the directions and magnitudes of the horizontal contemporaneous with the Upper Pontus sedimentation. The 0 1997 RAS, GJI 131, 61-86 ~· ~ ~ .., ~ 0:1 9 00 N trace, trace. fault ' fault ,, active the ·•,:;, '·I active to '\ the ... point '\ follows ::t50m ' arrows · \ :. white • ( Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 approximately Small • line . . basin). black ' '\ i the f.. Havza ' the \ where of (a) . f . o ., ', (northwest Sketch (b) . Valley . <;::ay offsets. ' ~-{~ tream s Istavlos the in right-lateral . ~~"" Koprubasi indicate at .( ':' . arrows : escarpment black · ' ,, - fault •. and - , ts ce . NAF '·:·:: fa - · the , of . triangular view - · of >i;.;:.~:: top ./-·.··q~··. -~; the -----· to "', point .,;. I'· North-northeast-facing (a) arrows ,:(J;2 , 12. white large Figure ~ ~ ~ ....., a, ..... 00 a, e © w "' "' I - - Y' Stress changes along the North Anatolian Fault Zone 83 I along the upper-crust-scale fault may not directly reflect the BLACK I flow in the ductile lithosphere, and is localized along a fault zone with a low degree of distributed deformation. This observation agrees with results deduced from palaeomagnetic studies (Platzman et a/. 1994; Tatar et a/. 1995). Indeed, around the current study area palaeomagnetic measurements indicate that there have not been measurable rotations on a regional scale associated with Plio-Quaternary dextral strike-slip motion along the NAF. Only small perturbations in the horizontal stress axes were observed locally (e.g. sites 46-49 and 33, Figs 9a and b). They result from small-scale block rotations within narrow zones of intense deformation in or adjacent to the fault zones. Figure 13. Sketch of the present-day far-field versus near-field stress regime around the NAF in the Anatolian block (transtension south of Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 the NAF approximately indicated by dotted area) and in the Eurasian Sources of stress-regime change and stress-field variation Pontide block (transpression north of the NAF approximately indi- in north Anatolia cated by plus signs). E-DR represents the displacement rate of North Anatolia east of the NAF bend, and W-DR, the displacement rate of The extrusion of the Anatolian block and the dextral movement North Anatolia west of the NAF bend. along the NAF are considered to be the direct consequence of the northward indentation motion of the Arabian plate. In regime change thus occurred during the Pleistocene, probably this context, the NAF constitutes the northern boundary of after the early Pleistocene, i.e. after the Pontus Formation the westward-moving Anatolian block and connects the com- sedimentation. As the orientations of the principal stress axes pressional regime in east Anatolia to the extensional regime in apparently remained fairly constant for both stress states Aegea. Spatial geodetic measurements show that the transfer (coaxial stress states, that is the NE-trending minimum hori- of motion from Arabia to Anatolia appears to be nearly zontal stress, 03,and NW-trending maximum horizontal stress, complete and thus that forces applied at the Anatolia/Arabia ol),they can be represented on a Mohr’s circle (Fig. 10). This boundary play a major role (Fig. 1). However, the southwest- representation shows that the change between these stress ward velocity increase of Anatolia indicates that the Aegean states corresponds to an R-value decrease (Fig. 1Oc) and results tectonics and consequently the Hellenic arc subduction prob- from a temporal decrease in both horizontal stress magnitudes ably influence Anatolian tectonics. Consequently, as previous (Fig. lOd), oHmaxdecreasing more than ohmin.This stress- studies did not record any noticeable change in the kinematics magnitude decrease induced a regional transtensional regime of eastern Anatolia during the time span studied, stress-regime which produced extensional deformation in the southwestern changes in central and western Anatolia may have resulted zone of the Central NAF convex bend, this deformation being from tectonic changes in Aegea. Detailed analyses of the recent due to local extension characterized by a NE-trending 03,that tectonics in Aegea have indicated that stress patterns have is parallel to the regional transtensional 03.The transtensional changed several times since the Upper Miocene (Mercier et al. character of the recent stress regime explains the compatibility 1979, 1987, 1989; Angelier et al. 1982; Sorel et al. 1988; Sorel between the observed normal and strike-slip faulting. Both er al. 1992). During most of the Pliocene-Lower Pleistocene, Quaternary stress regimes produced dextral shear along the the stress state was NE-trending extension in western Anatolia NAF and thus contributed to the extrusion of Anatolia. and northern Aegea. This period corresponds with one of However, a change in stress regime may induce a fault-slip- extensional tectonics within the central NAF bend (Over 1996). rate variation which parallels temporal strain-field changes, Since the Middle Pleistocene, a second extension phase has thus the change in the strike-slip stress regime can explain the occurred, characterized by NNE-trending extension in western discrepancy between the ‘short-term’ lateral slip rate deter- Anatolia and northern Aegea. This extension is still active in mined by geodetic measurements and the ‘long-term’ lateral western Anatolia, as shown by focal mechanisms (Fig. 3). The slip rate expected from geological studies. Indeed, the NAF change between the NE- and NNE-trending extensional lateral displacement rate estimated from offsets of geological regimes occurred during the late Lower Pleistocene or the and geomorphic structures (that is the average slip rate for the early Middle Pleistocene, between about 1 and 0.7 Ma (Sorel Miocene-Plio-Quaternary period) is 5-10 mm yr-’ (Barka et al. 1992). This period was characterized by a compressional 1992), about three times lower than the average slip rate given regime in the westernmost Aegean Arc (Sorel et al. 1988, 1992; by geodesy, which is 25 mm yr-’. Mercier et al. 1989) that could correspond to the trans- The trends of the horizontal stress axes for both old trans- pressional regime acting in the central NAF region. pressional and modern transtensional regimes obtained from Unfortunately, the timing of the temporal variations in stress the strike-slip striae as well as from earthquake slips are state inferred from the geological data collected around the relatively consistent throughout the region studied, that is over Central NAF is not sufficiently well constrained to ascertain distances of about 300 x 60 km. These remarkably homo- whether there has been a recent absolute change in the stress geneous stress fields characterized by stable stress strikes state or whether this variation was due to fluctuations over a indicate that the stress regimes determined are regionally long period. However, the Pontus sedimentation timing con- significant and permit the suggestion that regionally large- strained by our stratigraphic control allows us to suggest that scale block rotation did not occur in the central NAF region the change from a transpressional to a transtensional regime during the Quaternary. This indicates that the deformation occurred after the early Pleistocene in the central NAF zone. 0 1997 RAS, GJI 131, 61-86 84 0. Bellier et al. Thus, it could be related to the drastic tectonic-regime change to Prof. Z. Tutkun and Prof. A. Oztiirk who provided docu- in Aegea that occurred between 1 and 0.7 Ma. ments and expertise on the active faulting of the North Global models (DeMets et ul. 1990) suggest that the Hellenic Anatolian Fault and to D. Sorel and M. Sebrier for helpful trench is moving southwards relative to Europe as a conse- reviews and criticisms. We thank E. Carey-Gailhardis for quence of Mediterranean slab retreat, which produces present- helpful discussions concerning the calculated mean deviatoric day extension in the overriding plate, that is in the Aegean stress tensor and the stress-ratio significance in interpreting domain. However, in Aegea stress-regime changes explained inversion. We also thank G. Roche and L. Daumas for drawing by variations of the Mediterranean slab-pull force occurred the figures. during the Plio-Quaternary (Sorel et ul. 1988), the slab-pull force being considered as a major force in driving the plates and controlling the tectonics on an overriding plate above a REFERENCES subduction zone (e.g. Forsyth & Uyeda 1975; Stbrier & Soler 1991). A strong pull force related to the increase of the slab Altun, I., Sengun, M., Keskin, H., Akqaoren, F., Sevin, M. Deveciler, length (Forsyth & Uyeda 1975; Sorel et al. 1988) is generally 0. & Akat, U., 1990. Turk. Geol. 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Marmara Sea region, NW Anatolia, inferred from GPS measure- 17 40" 51.5' 33" 19.0' P ments, Grophp. Rex Lrtt., 22, 2533-2536. 18 40" 55.5' 33" 15.1' Cr Tapponnier, P., 1977. Evolution tectonique du systeme alpin en 19 40" 50.3' 33" 19.0' M Mediterrante: poinGonnement et ecrasement rigide-plastique, Bull. 20 40" 50.0' 33" 19.2' M Soc. Gkol. Frunce, 7, 437-460. 21 40" 49.3' 33" 19.1' M Tatar, O., Piper, J.D.A., Park, G.R. & Gorsoy, H., 1995. Paleomagnetic study of block rotations in the Niksar overlap region of the North 22 40" 50.1' 33" 21.0' P Anatolian Fault Zone, central Turkey, Tectonophysics, 244,251-266. 23 40" 51.5' 33" 23.7' P Taymaz, T., Jackson, J. & McKenzie, D., 1991. Active tectonics of 24 40" 53.6' 33" 25.0' PI+H north and central Aegean sea, Geophys. J. Int., 106, 433-490. 25 40" 54.1' 33" 24.5' Cr Vasseur, G., Etchecopar, A. & Philip, H., 1983. Stress state inferred 26 400 53.1' 33" 30.5' P from multiple focal mechanisms, Ann. Geophys., 1, 291-298. 27 40" 53.7' 33" 31.3' P-PI Watson, G.S., 1960. More significance tests on the sphere, Biomtrika. 28 40" 53.9' 33" 36.8' P-PI 47, 87-91. 29 40" 56.1' 33" 32.8' Cr Westaway, R., 1994. Present-day kinematics of the Middle East and 30 40" 53.1' 33" 37.8' Jr-Cr eastern Mediterranean, J. geophys. Res., 99, 12 071-12 090. 31 40" 58.3' 33" 37.5' Cr Wickens, J.W. & Hodgson, J.H., 1967. Computer re-evaluation of Downloaded from https://academic.oup.com/gji/article/131/1/61/752552 by guest on 01 October 2021 earthquake mecanism solutions, Dept Energy, Mine and Resources, 32 40" 56.9' 33" 44.3' Cr Ottawa, Canada. 33 41" 00.5' 33" 47.0' Jr-Cr Zanchi, A. & Angelier, J., 1993. Seismotectonics of Western Anatolia: 34 40" 54.3' 33" 48.5' M regional stress orientation from geophysical and geological data, 35 40" 54.3' 33" 48.8' M Tectonophysics. 222, 259-274. 36 40" 55.4' 33" 49.3' M 37 40" 55.2' 33" 50.0' M 38 40" 56.8' 34" 03.4' P-PI APPENDIX A. Location of fault striae measurement 39 41" 00.2' 34" 02.3' P-PI sites in latitude (lat.) and longitude (long.) and ages of 40 41" 00.2' 34" 02.3' P-PI faulted formations. Faulted-formation ages are as follows: 41 41" 00.2' 34" 02.5' P-PI H, Holocene; P-PI, undifferentiated Plio-Pleistocene (Pontus 42 41" 05.5' 34" 03.0' P-PI Group); PI, early (?) Pleistocene (Upper Pontus); P, Pliocene 43 41" 08.5' 34" 03.2' Jr-Cr (Lower Pontus); M, Miocene; Pa, undifferentiated Palaeogene; 44 41" 15.0' 34" 01.9' Jr-Cr Jr-Cr, undifferentiated Jurassic and Cretaceous; Cr, Cretaceous; 45 41" 02.7' 34" 11.5' Jr-Cr Jr, Jurassic. '+ ' indicates that two formations are faulted. 46 41" 06.1' 34" 10.9' P-P1 References used to date faulted formations are as follows: 47 41" 05.6' 34" 09.5' P-P1 Irrlitz (1972); Barka & Hancock (1984); Barka & Gulen 48 41" 06.3' 34" 12.1' P-PI (1988); Altun et al. (1990); Andrieux et al. (1995); Over 49 41" 06.4' 34" 18.2' P-PI (1996). 50 41" 06.6' 34" 12.2' Jr-Cr 51 41" 06.5' 34" 12.8' Jr-Cr Site Lat. (N) Long. (E) Age 52 41" 04.9' 34" 15.8' PI-H 53 41" 05.5' 34" 16.5' PI-H 1 40" 52.5' 32" 39.5' Cr 54 41" 06.3' 34" 20.0' Jr-Cr + PI 2 40" 52.6' 32" 41.3' Cr 55 41" 05.9' 34" 22.0' Jr-Cr 3 40" 50.8' 32" 41.9' Cr 56 41" 07.8' 34" 34.0' P 4 40" 50.2' 32" 48.0' Cr 57 41" 06.7' 34" 46.0' P 5 40" 54.0' 32" 47.9' Pa 58 41" 05.1' 34" 46.0' Jr-Cr + Pa 6 40" 53.5' 32" 48.0' Pa 59 41" 07.0' 34" 48.0' Jr-Cr + Pa 7 40" 50.7' 32" 50.9' Cr 60 41" 06.6' 34" 59.6' Jr-Cr 8 40" 50.8' 32" 51.4' Cr 61 40" 02.2' 34" 32.0' Jr-Cr 9 40" 49.2' 32" 53.5' PI 62 41" 00.6' 35" 59.8' Jr-Cr 10 40" 55.1' 32" 54.0' Pa 63 41" 05.3' 35" 29.1' P 11 40" 50.9' 33" 09.66' P1 64 41" 08.2' 35" 30.3' Jr-Cr 12 40" 51.0' 33" 09.4' P1 65 40" 53.3' 35" 50.8' Jr-Cr 13 40" 53.3' 33" 07.9' Cr 66 40" 54.5' 35" 38.0' Jr-Cr 14 40" 50.3' 33" 18.6' M 67 40" 50.5' 35" 49.1' Jr-Cr 15 40" 50.5' 33" 18.7' P 68 40" 47.2' 36" 02.5' Jr-Cr 16 40" 51.8' 33" 15.1' P 69 40" 46.9' 36" 03.2' Jr-Cr 0 1997 RAS, GJl 131, 61-86