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ABSTRACT

This study presents the first paleostress results obtained from the faulted Quaternary deposit in Tehran area. Stress inversion of 167 fault-slip data and 57 tension gashes at 28 sites revealed threee stress stages. These stages indicate a clockwise rotation of stress. The oldest stress direction trending NW-SE has been responsible for the forming of folds and faults of A-formation (the oldest unit of Quaternary). The following directed stress N-S and NE-SW have affected only the overlaying younger unit (B-Formation). The timing of stress change from NW-SE to NE-SW (average azimuth 030 for Tehran region) needs more study. 

INTRODUCTION

Tehran is situated in an embayment in the mountain front filled with alluvial materials originating from the rise of the Alborz range. The different types of alluvial formations deposited along the southern flank of Alborz were first studied by Rieben [1,2,3]. According to this classification is the A-bed (Hezardareh Formation) is the oldest unit which belongs to plio-Pleistocene Conglomerate [3]. It has responded to the Quaternary tectonic  activity by folding and faulting and its study provides  us with a means of understanding the Quaternary  deformations of the Alborz.
Researches reaveled two different stress directions specially in Eocene formation  [4,5,6]. [6] noted, that a number of NW-SE faults, generally  of post-Eocene age is reactivated in Post-Pliocene. The Anti-Alborz is bordered by several NW-SE en echelon faults and dyke systems which indicate dextral  wrench movements [4].
After studing of folded and faulted alluviums of Tehran [4] has proposed a stress direction of  N-S to NE-SW responsible for the deformation in post - pleistocene time. 
Fault plane solution of earthquakes in southern margine of central Alborz is accosiated with left lateral strike-slip motion [7,8].
The aim of present study is to date the mentioned stress directions using known  stratigraphy of alluviums and faults cutting the various units of it. 

GEOLOGICAL SETTING

Tehran is at the root of the southern flank of the Alborz chain (with average altitude of 1300m) at the abrupt topographic boundary between the mountain range (Mt Tochal 3933m) and the northern border of the central Kavir Desert. The mountain range is formed by a series of folds which trend NW-SE and thrusts dipping to the North. These structures are mostly formed by the rocks of the Karaj formation, an Eocene submarine volcanic and pyroclastic complex. The northern boundary of this complex was limited with the Mosha-Fasham Fault which is a north-dipping fault and the North Tehran Fault is a branch of it [9].
The North Tehran fault is the boundary between the Karaj Formation and the alluvial deposits of Tehran.(Hezardarreh Formation)
To the southeast of Tehran, the Anti-Alborz Mountain form a separate block with a geological history unrelated to the main range. [10,11] The rocks forming this mountain consists mainly calcareous formations, but include sedimentary rocks from Devonian to lower Tertiary, and some volcanic and plutonic rocks. 

STRATIGRAPHY OF TEHRAN ALLUVIUM

The different types of alluvial formations were first studied by Rieben [1,2,3] and later autors have adopted his classifications [11,12,13,4].
In the present study we used the Rieben’s classification. The alluvial deposits contain four stratigraphic units beginning with A (oldest unit) to D (Youngest unit).

1. The oldest and most important of the four units. The Hezardarreh formation consists of materials that had source like the overlaying B and C units, mainly in the Eocene formation. The average grain size is between one centimeter to one decimeter. A characteristic of this ancient alluvium is the regularity of its bedding. Unregulary it displays beds of sandy gravel which is cemented by lime carbonate. An angular unconformity was observed by Rieben [3]. Further study by revealed three unconformity in A  (Hezardarreh ) formation. This unit is folded and dissected by numerous faults  (Fig. 1). 

2. This unit is made of sandy or clayey loam. the colore is pale brown and overlay the A beds with a angular unconformity. In contrast to A beds it is not consolidated. The size of pebels in a matrix of sand and clay differ considerably in dimensions.
   This heterogeneous formation overlay the surface of strongly eroded A-formation. The heterogenity of this formation was for Riviere [10] an indication for to take it as a moraine. The tilting of this unit exceed max 15 degree. The observation of an angular unconformity in B-beds leaded Rieben [3] to distinguish a lower and a upper B-formation.

3. This formation is distiguished by its red conglomerate custs of laterite [3]. It has regular stratification with a light deep from North Tehran to southern part of city. 

METHODOLOGY

The study of fault planes was carried out using analytical techniques with the  purpose to reconstruct the evolution of paleostress fields (following [14,15]). Assuming that the maximum shear stress on a fault plane is paralle to the observed striation, an average stress deviator can be computed minimizing the angle between the measured stration and the theoretical direction of the maximum shear stress, when at least four fault planes with different orientations are available. The algorithm proposed by  [16] and her computer program have been used in this work. Stress tensors were determined in 28 sites (Fig.5). About 57  tension gashes were also analyzed, using the relationships between stress and  simple brittle structures, suggested by [17]. In order to check the reliability of the results obtained from the analysis of faults (Hancock 1985). They were compared  with the kinematics of faults observed and mapped by aerial photos in scale of 1: 50000.

TYPE OF MEASUREMENTS

Field analysis of tectonic structures is a basic tool in the study of the stress field. At outcrop scale, structural observations such as striated fault planes and tension gashes can be used with numerical methods to infer the orientation and the relative magnitudes of the stress tensor [18,19,20,21,22]. 
The Orientation of striae contained on fault surfaces has been determined for167 faults. This striations are well preserved on the fault surfaces which cut the layers containing a high percent of clay. In some outcrops however it is difficult to determine the sense of movements. 
We have used some criteria for determining the sense of slip on faults as following:
If striae are observed on a fault that offsets planar elements, such as bedding, the sense of slip can be estatilished. 
In homogeneous units, spacially A-bedds, lacking marking layer are objects such as pebbles that are identified across a fault, the sense of slip are established.
Numerous small fractures that we identify as Riedel shears generally splay into the fault zone from the main fault plane . Their trace on, and their angle of, intersection with the main fault surface are easy to detect; these Riedel shear fractures make angels of  with the main fault. These small features and their geometry enables one to determine the sense of relative movements on some faults. The sense of motion is opposite to that which would be determined by contrasts in roughness felt as one rubs his hand back and forth, parallel to straie.
In some few cases is the sense of a known fault section is determined and confirmed by geomorpholigical evidences by aerial photos. 

FAULT TYPES AND ORIENTATION

Where outcrop conditions permit, analysis of fault data sets were carried out both in terms of fault-slip geometries and mechanisms and related paleostresses.
Although the number and locations of our sites can not be considered representative for a regional distribution of faults orientations, some major characteristics are easily derived from the rose diagrams (Fig.2) some 167 faults with slickenside lineations were observed  at 28 sites (Fig.5).
Figure (4) illustrates the distribution of fault types in our entire fault data set through a simple analysis of the relationships between dip of fault planes and plung of fault straie. The entire data is grouped in two main sets; first set containing dip-slip movements (Fig.4c) and second set containing strike-slip movements (Fig.4d).
Reverse and normal faults (Fig.4c) are mainly high-angle faults which show dip-slip movements with minor amount of horizental component.  Oblique-slip movements paly a minor role a though they are present in the data set.
The distribution of normal and reverse fault strikes is shown in two rose diagrams (Figs 2a, b,c,d). For reverse faults, there is a dominant E-W trend (Fig. 2b).
The rose diagram of normal fault strikes shows three major subgroups:NE-SW, NW-SE and NNW-SSE (Fig.2a). The orientation of a normal faults are in good agreement with the studied area .
Strike-slip faults are divided in two sets according to the sense of their motion namely, left lateral (Fig.2d) and right lateral (Fig .2c) faults.
Left lateral faults strike between azimuth 020-070 and another set strike between 280-390. A minor set of left lateral fault strike between 320-360 (Fig.2d).
Right lateral faults show also three subsets (Fig .2c). The major population strike between azimuths 320-340. The second azimuth 075. For the minor set there is a NNE-SSW trend (average azimuth 020).
As it is apparent the strike-slip faults beare three different orientation which couldn’t be consistent with only one single direction of stress. 

STRESS HISTORY

A lot of superposition of striae founded on the same fault surface could hardly be created during a single and continous deformation process. Thus they indicate marked changes in the overall stress field orientation.
The various orientation of  is demonstrated in Figure 5. The oldest stress direction (NW-SE) determined in A-Formation was responsible for formig of thrust faults. with a minor right lateral component (e.g. shlan-kosar, mahmudieh and davudieh).
After this stage are two stress orientation (N-S and NE-SW) associated with the strike-slip faults. These orientations was measured in B-formation.

CONCLUSION

The Quaternary stress field derived from the interpretation of structural observations (faults and tension gashes) in Tehran area shows  a distinct rotation pattern with time. Different sites show similar rotation of principal stress axes. 
The stress-field seems to rotate clockwise until  points to the NE-SW in late Quaternary (Holocene ?). This stress-field rotation is due to the high activity of the area. According to the present day stress direction in Tehran area (average azimuth 30ú) is Niavaran fault suitably oriented to accommodated large horizontal movement. 

REFERENCES

• Rieben, E.H., 1953a. Note Preliminaries sur lest terrains alluviaux de Teheran et particulierment du territoire de Shemran. Bull. Lab. Geol. Min. Geophy. et Mus. Gel. (Univ. Lausanne), 105, 1-12.

• Rieben, E.H., 1061. Rieben, E.H., 1955. The geology of the Tehran Plain. Am. J. Sci. 253, 617-639.

• Rieben, E. H., 1966. Geological observations on alluvial deposits in northern Iran. Geol. Surv. Iran. 9, 39p.

• Tchalenko, J. S., Berberian, M., Iranmanesh, H., Bailly, M., and Arsovsky, M. 1974. Tectonic framework of the Tehran region. In: Materials for the study of seismotectonics of Iran; North-central Iran. Geol. Surv. Iran. 29, 7-46.

• Dellenbach, J., 1964. Contribution a I’etude geologique de region situess al’est de Tehran (Iran): Fac. Sci., Univ. Strasbourg (France), 117p.

• Berberian, M., 1971. Preliminary Report on structural Analysis of Ipak active fault - Geological survey, internal Report.

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• Jackson, J. and Mc Kenzie, D., 1984. Active tectonics of the Alpine-Himalayan Belt between western Turkey and Pakistan. Geophys. J. R. astr. Soc., 77, 185-264.

• Tchalenko, J. S., 1975. Seismotectonic framework of the North Tehran fault. Tectonophysics, 29, 411-420.

• Riviere, A., 1934. Contribution a l etude geologique de l Elburz (Perse). Rev. Geogr. Phys. et Geol. Dynam., Paris, 7(1-2), 194p.

• Engalenc, M., 1968. Contribution a la Geologie, Geomorphologie, Hydrogeologie de la region de Tehran (Iran). C.E.R.H., Montpellier, France, 365p.

• Vita-Finzi, C., 1969. Late Quaternary alluvial chronology of Iran. Geol. Rdsch, 58, 951-973.

• Bassir, M., 1971. Ingenieurgeologische Baugrund Untersuchungen in der Region Gross-Tehran, Iran. Dissertation, Rheinisch-Westfalische. Tech. Hochsch., Achen, 190p. 

• Angelier, J. 1984. Tectonic analysis of fault slip data sets. J.geophys. Res. 89, 5835-5848.

• Angelier, J., Colletta, B. & Anderson, R. E. 1985. Neogene paleostress changes in the Basin and Range: a case study at Hoover Dam, Nevada-Arizona. Bull. geol. Soc. Am. 96, 347-361.

• Delvaux, D., 1993. The TENSOR Program for reconstruction: examples from the East African and the Baikal rift zones. Terra Abstracts. Tect. Ann. 5: 216.

• Hancock, P. 1985. Brittle microtectonics: principles and practice. J. Struct. Geol. 7, 437-457.

• Carey, E. & Brunjer, B., 1974. Analyse theorique et numerique d’un modele mecanique elementaire applique a letude d’une population de failles, C. R. hebd. Seanc. Acad. Sci. Paris, 279, 891-894.

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• Gephart, J. W. & Forsyth, D.W., 1984. An improved method for determining the regional stress tensor using earthquake focal mechanisms, J. Geophys. Res., 97, B8, 11821-11827. 

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Fig.1. Geological map of Tehran area and Faults dissecting alluvium various ages.
A-Formation (Plio-Pleistoccene),  D-Formation (Recent)
A: Abbas-abad, B: Bagh-e-F-feyz, D: Davoudieh, D1*, D2*: Daneshgah, G: Golestan, 
D1: Darakeh(East &West)*. GH: ghazal, J1*, J2*:Jam-e-jam
K*:Kan, Ko: Kosar-shian, L: Lashgarak, M: Mahmoudieh, Mk*: Mekanir, Niavaran,
P:Punak, Pr*:Pardisan, S:Sohanak, Sh*: Shahran, Sd*: Saadat, T: Telo, Tv:Television, 
V: Vanak( *: presented by this study)

Fig.3. Diagramatic representation of strike-slip versus dip-slip component of fault offsets
for 167 faults  with slickenside lineations  and known sense of motion.

Fig. 4. Dip of fault plane versus dip of fault straie indicating the fault type 
(dip-slip, strike-slip, or obliques slip fault). a)explanation, (b)all faults,
(c) normal & reverse faults (d) strike-slip faults (sinstral & dextral)