Antoine Coutrot
email: antoine.coutrot@gipsa-lab.grenoble-inp.fr

Nathalie Guyader

Gelu Ionescu

Alice Caplier

Video viewing: do auditory salient events capture visual attention?

Keywords: saliency, eye movements, sound, videos, attention, multimodality, audio visual

We assess whether salient auditory events contained in soundtracks modify eye movements when exploring videos. In a previous study, we found that on average, non-spatial sound contained in video soundtracks impacts on eye movements. This result indicates that sound could play a leading part in visual attention models to predict eye movements. In this research, we go further and test whether the effect of sound on eye movements is stronger just after salient auditory events. To automatically spot salient auditory events, we used two auditory saliency models: the Discrete Energy Separation Algorithm and the Energy model. Both models provide a saliency time curve, based on the fusion of several elementary audio features. The most salient auditory events were extracted by thresholding these curves. We examined some eye movements parameters just after these events rather than on all the video frames. We showed that the effect of sound on eye movements (variability between eye positions, saccade amplitude and fixation duration) was not stronger after salient auditory events than on average over entire videos. Thus, we suggest that sound could impact on visual exploration not only after salient events but in a more global way.

one visually explores its surroundings, the regions that are the most likely to attract attention are called salient regions. During the last decades, the modeling of saliency has been a very active field of research, from neurosciences to computer vision. Saliency models rely on the detection of spatial locations where the local properties of the visual scene such as color, motion, luminance, edge orientation... significantly differ from the surrounding image attributes [START_REF] Itti | A model of saliency-based visual attention for rapid scene analysis[END_REF] [START_REF] Marat | Modelling Spatio-Temporal Saliency to Predict Gaze Direction for Short Videos[END_REF] [START_REF] Treisman | A feature-integration theory of attention[END_REF]. Saliency models are evaluated by comparing the predicted salient regions with the areas actually looked at by participants during eye-tracking experiments. Being able to predict the salient regions of an image or a video leads to a multitude of applications. For instance, saliency-based video compression algorithms are particularly efficient [START_REF] Itti | Automatic Foveation for Video Compression Using a Neurobiological Model of Visual Attention[END_REF]. For each video frame, these algorithms encode the salient areas with a better resolution than the rest of the scene. Since one perceives only a small area around the center of gaze at high resolution (the fovea, around 3 ˚of visual angle), the distortion of non salient regions does not impact the perceived quality of the visual stimulus [START_REF] Cater | Detail to attention: exploiting visual tasks for selective rendering[END_REF] [START_REF] Li | Visual attention guided bit allocation in video compression[END_REF]. Another application of saliency models is the automatic movie summarization [START_REF] Evangelopoulos | Video event detection and summarization using audio, visual and text saliency[END_REF]; video summary contains the most salient frames spotted by a visual attention model. The increasing availability of video bases implies a growing need for powerful indexation tools: automatically extracting the most salient frames of a video is an efficient way to evaluate its relevance. Saliency models also exist (although to a smaller extent) for audio signals. Auditory saliency models have been developed to detect the prominent syllable and word locations in speech [START_REF] Kalinli | A saliency-based auditory attention model with applications to unsupervised prominent syllable detection in speech[END_REF] or to automatically annotate music with text tags (music style, mood, speech...) [START_REF] Tingle | Exploring automatic music annotation with "acoustically-objective" tags[END_REF]. The existence of a strong interaction between vision and audition is well known, as reflected by the numerous audiovisual illusions [START_REF] Mcgurk | Hearing lips and seing voices[END_REF] [START_REF] Vroomen | Sound Enhances Visual Perception: Cross-modal Effects of Auditory Organization on Vision[END_REF]. Previous studies showed that sound modified the way we explore visual scenes. For instance, a spatialized sound tends to attract gazes toward its loca-tion [START_REF] Corneil | The influence of auditory and visual distractors on human orienting gaze shifts[END_REF]. Onat and colleagues presented static natural images and spatially localized (left, right, up, down) simple sounds. They compared eye movements of observers when viewing visual only, auditory only or audio-visual stimuli. Results indicated that eye movements were spatially biased towards the regions of the scene corresponding to the sound sources [START_REF] Onat | Integrating audiovisual information for the control of overt attention[END_REF]. Still, the combination of visual and auditory saliency models has rarely been investigated. Moreover, when used on videos, saliency models never take into account the information contained in the soundtrack. When running eyetracking experiments with videos, authors do not mention soundtracks or explicitly remove them, making participants look at silent movies which is far from natural situations. Our aim is to assess whether an auditory saliency model based on physical characteristics of the signal can be used to examine the impact of sound on observers' gaze while watching videos. Here, we do not focus on sound spatialization but simply on salient auditory events that might reinforce the saliency of visual events. In a previous study, we showed that soundtracks do have a global impact on visual exploration when watching videos [START_REF] Coutrot | Influence of soundtrack on eye movements during video exploration[END_REF]. In this study, we go further and examine whether this impact is stronger just after salient auditory events. For that purpose, we spotted salient auditory events in video soundtrack using two models. First, a popular auditory saliency model, the "Discrete Energy Separation Algorithm" (DESA). Second, a simple energy-based model. We analyzed the results of an eyetracking experiment in which we recorded gazes of participants watching videos with and without their related soundtrack. First, we present the results obtained in [START_REF] Coutrot | Influence of soundtrack on eye movements during video exploration[END_REF], where we tested the general impact of sound on eye movement parameters. We found that observers looking at videos with their soundtracks had different eye movements than observers looking at the same videos without sound. Second, we focus on sound impact on eye movements following auditory salient events spotted by a model. A founding rule of multisensory integration is the temporal rule: multisensory integration is more likely or stronger when the stimuli from different modalities are synchronous [START_REF] Stein | The merging of the senses, mit press edn[END_REF]. This rule has been established by comparing the electrical activity of some neurons when presenting simple visual stimuli (light flashes) with or without synchronous or delayed simple auditory stimuli (bursts). Studies showed that neurons activity was much stronger in multimodal than in unimodal condition, and that this reinforcement was maximal for synchronous stimuli [24] [25]. Here, we generalize this idea to more complex stimuli by identifying bursts to auditory saliency peaks and light flashes to the corresponding visual information. Thus, we compare the eye movements made over whole videos to those made over the few frames following auditory saliency peaks. It has been shown that audio and visual stimuli can be judged as synchronous across a broad range of physical offsets, typ-ically in a 400 ms temporal window (see [START_REF] Recanzone | Interactions of auditory and visual stimuli in space and time[END_REF] for a review). This flexibility is probably due to the different propagation velocity between modalities in the environment (light: 300 000 km/s; sound: 0.34 km/s) and in the human body (conduction time from the retina to the brain: around 10 ms [START_REF] Gouras | The effects of light-adaptation on rod and cone receptive field organization of monkey ganglion cells[END_REF]; from the cochlea to the brain: around 50 ms [START_REF] Kraus | Electrophysiology of the human auditory system[END_REF]). Moreover, this window seems to be flexible with regard to input type. Complex stimuli are easier to integrate than simple ones, thanks to prior experience: one is more used to associate speech with moving lips or thunder with lightning than simple bursts with light flashes [START_REF] Guski | Audiovisual phenomenal causality[END_REF]. Thus, the temporal window during which a salient auditory event might significantly interact with visual information is around 400 ms but is not precisely determined. That is why in this research we chose to compare the eye movement parameters made over whole videos vs. the ones made over the 5 (200 ms), 10 (400 ms) and 25 (1s) frames following saliency peaks.

To summarize, the main goal of this study is to test whether the global effect of sound that was previously found on eye movements is reinforced just after salient audio events. The salient events are emphasized through two models: the DESA and the Energy models. We compared some eye movement parameters (the dispersion between eye positions, the mean saccade amplitude and the mean fixation duration) recorded on videos seen with and without their original soundtracks. The comparison was done over whole videos vs. over the few frames following salient audio events. To discuss our results, we ensured through an additional experiment that the salient audio events spotted by the models are effectively judged as more salient by listeners than random events.

Auditory saliency models

Attention, both in visual and auditory modalities, is mainly caught by features standing out from their background (e.g. motion, bright colors or high intensities). In a complex scene, the auditory system segregates sounds by extracting features such as spectral or temporal modulations [START_REF] Bregman | Auditory Scene Analysis, the perceptual organization of sound[END_REF]. In this section, we describe the two models used to spot auditory salient events in soundtracks. First, the Discrete Energy Separation Algorithm (DESA) is detailed. This algorithm has recently been brought forward in many fields of research involving the detection of auditory information, such as movie summarization or speech analysis [START_REF] Evangelopoulos | Multiband Modulation Energy Tracking for Noisy Speech Detection[END_REF] [START_REF] Evangelopoulos | Video event detection and summarization using audio, visual and text saliency[END_REF]. Second, we present a model merely based on the signal energy.

Discrete Energy Separation Algorithm

Even if our understanding of auditory saliency is still limited, previous studies had shown that extracting amplitude

max i∈[1..N ] (M T E i ) M T E 1 = 1 L L k=1 Ψ (s 1 (k)) M T E N = 1 L L k=1 Ψ (s N (k)) Ψ (s 1 (k)) Ψ (s N (k))

Mean Teager Energy

Amplitude Frequency

Mean Teager Amplitude

Mean Teager Frequency

Bandpass Filter Teager-Kaiser Energy

Average over the audio samples of the frame

s(k) (k = audio sample) s 1 s N
Fig. 1 Discrete Energy Separation Algorithm processing stages on an auditory signal s split into N frequency bands. Teager-Kaiser Energy, given by equation 1, is averaged over all audio samples k contained in a frame (there are L = 48000 x 0.04 = 1920 audio samples in a 40 ms audio frame sampled at 48 kHz). We chose the frequency band with the maximal Teager-Kaiser Energy (MTE). The Mean Teager Amplitude (MTA) and Frequency (MTF) are computed from the Teager-Kaiser Energy thank to equations 2 and 3. The three features are then combined to compute the auditory saliency value of the frame.

and frequency modulations is essential to predict the natural orienting behavior of humans to audio signals [9] [19].

The Discrete Energy Separation Algorithm (DESA) is an auditory saliency model based on the temporal modulation of amplitude and frequency in multiple frequency bands.

The multiband demodulation analysis allows the captures of such modulations in the presence of noise, which is often a limiting factor when dealing with complex auditory scenes [START_REF] Bovik | AM-FM Energy Detection and Separation in Noise Using Multiband Energy Operators[END_REF]. The DESA is simple and efficient. The process applied to each audio frame is described in Figure 1. The input signal s is separated in several frequency bands thank to Gabor filters. A Gabor filter is described in time as

h i (t) = exp(-α 2 i t 2
)cos(ω i t) with (ω i , α i ) respectively the central frequency and the filter bandwidth (i ∈ [1..N ] with N the total number of filters) [START_REF] Bovik | AM-FM Energy Detection and Separation in Noise Using Multiband Energy Operators[END_REF]. Their placement and bandwidth have been chosen such that two neighboring filters intersect at half-peak.

ω i = 3Ω c 2 i+1 α i = ω i 2 √
ln 2 with Ω c the highest frequency to be analyzed. Concretely, the video soundtracks were sampled at 48 kHz and separated in six frequency bands respectively centered on ω i ∈ {281, 562, 1125, 2250, 4500, 9000} Hz. This spectrum covers a broad type of audible noises (e.g. speech: from 50 Hz to 8 kHz). Given an audio sample k, the Teager-Kaiser energy is computed for each frequency band: Teager-Kaiser energy:

Ψ [s[k]] = s 2 [k] -s[k + 1]s[k -1] (1)
The Teager-Kaiser energy is prized for its ease of implementation and its narrow temporal window, making it ideal for local (time) analysis of signals.The Teager-Kaiser energy is often used for detecting amplitude and frequency modulations in AM-FM signals [17] [31]. To separate the noise from the signal of interest, the frequency band in which the Teager-Kaiser energy is maximal is selected. In this frequency band, we separate the instantaneous energy into its amplitude and frequency components, according to the following equations.

Instant amplitude:

|a[s[k]]| = 2 Ψ (s[k]) Ψ ( ṡ[k]) (2)
Instant frequency:

f [s[k]] = 1 2π arcsin Ψ [ ṡ[k]] 4Ψ [s[k]] (3)
with ṡ the derivative of the signal. Each feature is averaged over a number of audio samples k corresponding to a frame duration (40 ms), to compute the mean Teager energy (MTE), the mean instant amplitude (MIA) and the mean instant frequency (MIF). The MTE, MIA and MIF are then normalized and combined to compute the auditory saliency value S of the current frame m.

Here, we averaged the three features:

S(m) = w 1 MTE(m) + w 2 MIA(m) + w 3 MIF(m)
with

w 1 = w 2 = w 3 = 1 3
Since different weighing could lead to different results, one could adapt it according to the mean value of each feature.

If the sound to be analyzed contains great energy variations (e.g. an argument with many raised voices), one is likely to give a preferential weighting to the MTE. On the contrary, if the sound contains great frequency variations (e.g. moving police siren presenting Doppler effect), the MIF will be preferred. Here, we chose an equally weighted combination to make the DESA as flexible as possible. Figure 2 illustrates the DESA algorithm applied to an audio signal (first plot). Three features were computed and averaged for each frame to provide the three curves MTE, MIA and MIF. Finally, the saliency curve was computed by averaging the three upper curves. Thresholding this auditory saliency curve gave the signal "saliency peaks" (vertical red bars). We normalized the number of saliency peaks over time. First, we chose a rate of one peak for two seconds: a N-second long signal had N/2 saliency peaks. Second, the time interval between two peaks had to be longer than one second: two neighboring peaks were distant enough so that the potential effect they might induce did not affect each other.

Energy model

We compared the peaks given by our auditory saliency model based on DESA algorithm, we also computed saliency auditory peaks using the energy curve (second plot of Figure 2), given by

E[s[k]] = s 2 [k]
We extracted the "Energy peaks", i.e. the local maxima of the energy curve, at the same rate as saliency peaks (one peak for two seconds and at least one second between two peaks).

We used these two sets of peaks ("DESA peaks" and "Energy peaks") to evaluate the impact of sound on eye movements recorded during the eye-tracking experiment described below.

Methods

To observe the impact of salient auditory events on eye movements while freely watching videos, we set up an eye-tracking experiment. We built a base of 50 videos and asked 40 participants to watch it, half with and half without its related soundtracks. The experimental set-up and the data presented here were used in a previous paper [START_REF] Coutrot | Influence of soundtrack on eye movements during video exploration[END_REF].

Apparatus

Eye-movements were recorded using an Eyelink 1000 eyetracker (SR Research). We used the eye-tracker in binocular "pupil -corneal reflect" tracking mode. Eye positions are sampled at 1 kHz with a nominal spatial resolution of 0.01 ˚of visual angle. The device is controled by the software SoftEye [START_REF] Ionescu | SoftEye software[END_REF] that allows to control stimuli presentation. Participants were sat 57 cm away from a 21 inch CRT monitor with a spatial resolution of 1024 x 768 pixels and a refresh rate of 75 Hz. Head was stabilized with a chin rest, forehead rest and headband. Soundtracks were listened using headphones (HD280 Pro, 64Ω, Sennheiser).

Participants and stimuli

Participants 40 persons participated at the experiment: 26 men and 14 women, aged from 20 to 29 years old. All participants had normal or corrected to normal vision and hearing and were French native speakers. They were naive about the aim of the experiment and were asked to watch videos freely. We discarded data from 4 participants due to recording problems.

Stimuli We used 50 video clips extracted from professional movies as varied as possible, to reflect the diversity of audiovisual scenes that one is likely to see and hear (dialogue, documentary film, drama, action movies). When the soundtrack contained speech, it was always in French. Each video sequence had a resolution of 720 x 576 pixels (30 ˚x 24 ˚of visual angle) and a frame rate of 25 frames per second. They lasted from 0.9 s to 35 s (mean = 8.7 s; standard deviation = 7.2 s). Video sequences lasted overall 23.1 min. We chose video shots with varied durations to avoid any habituation effect from the participants: one could not predict when each stimulus ended. As explained in the introduction, we chose to focus on the influence of nonspatial sound on eye movements. Hence, we used monophonic soundtracks.

We analyzed the parameters of eye movements on average over each video shot rather than over entire videos. A shot cut is an abrupt transition from one scene to another that greatly impacts the way one explores videos [10] [START_REF] Smith | A Window on Reality: Perceiving Edited Moving Images[END_REF]. In a preliminary study [START_REF] Coutrot | Audio Tracks Do Not Influence Eye Movements When Watching Videos[END_REF], we elicited the effect of video editing (shots and cuts), studying the effect of sound over entire videos made up of several shots, and found no significant impact of sound on eye movements. However, this effect exists and has been observed when taking into account video editing, at least for shots longer than one second, which is the case of practically all the shots of our database [START_REF] Coutrot | Influence of soundtrack on eye movements during video exploration[END_REF]. Thus, in the present work, we did not study entire videos but we examined each shot. Shots were automatically detected using the mean pixel by pixel correlation value between two adjacent video frames. We ensured that the shot cuts detected were visually correct. Sequences contained different number of shots, with a total number of 163 shots.

Procedure

The experiment consisted in freely viewing 50 video sequences. The first 20 participants saw the first half of videos with their soundtracks and the other half without. This was counterbalanced for the last 20 participants. Each experiment was preceded by a calibration procedure, during which participants focused their gaze on 9 separate targets in a 3 x 3 grid that occupied the entire display. A drift correction was done between each video, and a new calibration was done at the middle of the experiment or if the drift error was above 0.5 ˚. Before each video sequence, a fixation cross was displayed in the center of the screen for 1 second. After that time, and only if the participant looked at the center of the screen (gaze contingent display), the video sequence was played on a mean grey level background. Between two consecutive video sequences a grey screen was displayed for 1 second (see Figure 3). Participants wore headphones during the entire experiment, even when the stimuli were presented without soundtrack. To avoid presentation order effects, videos were run randomly. At the end, each video was seen by 20 persons with its related soundtrack and by other 20 persons without its related soundtrack.

Data extraction

The eye tracker system gives one eye position each millisecond, but since the frame rate is 25 frames per second, 40 eye positions per frame and per participant were recorded. Moreover, we only analyzed the guiding eye of each subject.

In the following, an eye position is the median position that corresponds to the coordinates of the 40 raw eye positions: there is one eye position per frame and per subject. Eye positions corresponding to a frame during which participants made a saccade or a blink were discarded from analysis. For each frame and each stimulus condition, we discarded outliers, i.e. eye positions above ± 2 standard deviations from the mean. The eye-tracker software organizes the recorded movements in events: saccades, fixations and blinks. Saccades are automatically detected by the Eyelink software using three thresholds: velocity (30 degrees/s), acceleration (8000 degrees/s 2 ) and saccadic motion (0.15 degree). The velocity threshold is the eye movement velocity that must be exceeded for a saccade to be detected. Acceleration threshold is used to detect small saccades. The saccadic motion threshold is used to delay the onset of a saccade until the eye has moved significantly.

Fixations are detected as long as the pupil is visible and as long as there is no saccade in progress.

For each stimulus condition, we discarded outliers, i.e. saccades (resp. fixations) whose amplitude (resp. duration) was above ± 2 standard deviations from the mean. Moreover, we discarded data from four subjects due to recording problems. We separated the recorded eye movements in two data sets.

-The data recorded in the Audio-Visual (AV) condition, i.e. when videos were seen with their original soundtrack. -The data recorded in the Visual (V) condition, i.e. when videos were seen without any sound.

Results

In this section, we examine the eye movements recorded in Visual (V) and Audio-Visual (AV) conditions. We compare eye movements averaged over all the frames of a same shot and over the few frames following each "Energy peak" or each "DESA peak". The presented results are based on a 10-frame time period after Energy and DESA peaks. The same analysis was carried out on a 5 and 25-frame period and led to the same results (see Section 4.1), that we chose not to plot. We discarded from analyzes shots without DESA (resp. Energy) peaks (36 shots). We examine several parameters in both V and AV conditions: the dispersion between the eye positions of different observers, which reflects the variability between them; the amplitude of the recorded saccades and the duration of the recorded fixations.

Eye position dispersion

To estimate the variability of eye positions between observers, we used a measure called dispersion. For a frame and for n participants (thus n eye positions p = (x i , y i ) i∈[1.

.n] ), the dispersion D is defined as follow:

D(p) = 1 n(n -1) n i=1 n j=1 j =i (x i -x j ) 2 + (y i -y j ) 2
In other words, the dispersion is the mean of the Euclidian distances between the eye positions of different observers for a given frame. If all participants look close to the same location, the dispersion value is small. On the contrary, if eye positions are scattered, the dispersion value increases. In this analysis, we computed a dispersion value for each frame, in both V and AV conditions. First, we averaged dispersion over all frames of each shot. Then, we averaged dispersion over the 10 frames following each DESA and Energy peak. To control, we computed 1000 random sets containing "random peaks" at the same rate as Energy and DESA peaks. For each random set, we averaged dispersion over the 10 frames following each "random peaks" and took the mean of these 1000 "random" dispersion values. Results are shown in Figure 4. We first notice that in all cases, dispersion is smaller in Audio-Visual than in Visual Condition. To test the impact of sound on the dispersion values of the 127 video shots containing a DESA or Energy peak, we ran two analyzes of variance (ANOVAs). The first ANOVA was run with two factors: the stimulus condition (visual and audio-visual) and the window size (all frames and 10 frames) used to average dispersion after the Energy peaks. The second ANOVA was also run with two factors: the stimulus condition (visual and audio-visual) and the window size (all frames and 10 frames) used to average dispersion after the DESA peaks. For the first one, we found a main effect of sound (F(1,126) = 28.2; p < 0.001) and no effect of Energy peaks (F(1,126) = 0.82; n.s). We found similar results for the second one: a main effect of sound (F(1,126) = 24.8; p < 0.001) and no effect of DESA saliency peaks (F(1,126) = 1.52; n.s). To test the impact of the size of the averaging time window on the dispersion values of the 127 video shots containing a DESA or Energy peak, we ran two ANOVAs. The first ANOVA was run with two factors: the stimulus condition (visual and audio-visual) and the window size (averaged over 5, 10 and 25 frames after DESA peaks). We found a main effect of sound (F(1,126) = 31.8; p < 0.001) and no effect of the size of the averaging time window (F(2,124) = 0.9; n.s). The second ANOVA was also run with two factors: the stimulus condition (visual and audio-visual) and the window size (averaged over 5, 10 and 25 frames after Energy peaks). We also found a main effect of sound (F(1,126) = 39.6; p < 0.001) and no effect of the size of the averaging time window (F(2,124) = 0.2; n.s). To sum up, we found that the presence of sound does impact on gaze dispersion but neither the proximity of DESA or Energy peaks nor the size of the averaging time window affects this parameter.

Saccade amplitude and fixation duration

Figure 5 and Figure 6 respectively compare the average saccade amplitude and fixation duration in Audio-Visual and Visual conditions over all frames and over the ten frames following each DESA peak. For the sake of clarity, we omitted the saccade amplitude and fixation duration made after Energy and random peaks. The results are similar as the plotted ones. Distributions follow a positively skewed, long-tailed distribution, which is classical when studying such parameters during scene exploration [START_REF] Ho-Phuoc | When viewing natural scenes, do abnormal colours impact on spatial or temporal parameters of eye movements[END_REF] [START_REF] Tatler | The long and the short of it: Spatial statistics at fixation vary with saccade amplitude and task[END_REF]. We notice that participants tended to make smaller saccades and shorter fixations in V than in AV condition. We performed two analyzes of variance (ANOVA) with two factors (visual and audio-visual ; all frames and 10 frames after DESA peaks) on the 36 participant's median saccade amplitude and median fixation duration. For saccade amplitude, it revealed a main effect of sound (F(1,35) = 4.9; p = 0.033) and no effect of DESA peaks (F(1,35) = 0.27; n.s). For fixation duration, there was still no effect of DESA peaks (F(1,35) = 0.01; n.s) and the effect of sound was not significant (F(1,35) = 2.1; p = 0.15).

General discussion

We compared eye positions and eye movements of participants freely looking at videos with their original soundtracks (AV condition) and without sound (V condition). In a previous study, we showed that soundtrack globally impacts on eye movements during video viewing [START_REF] Coutrot | Influence of soundtrack on eye movements during video exploration[END_REF]. We showed that in AV condition, the eye positions of participants were less dispersed and tended to shift more from the screen center, with larger saccades. We also showed that observers did not look at the same locations, according to the viewing condition. An interpretation of these results is that sound might strengthen visual saliency. Indeed, with sound, observers explored videos in a more uniform way, leading to a decrease of the dispersion between eye positions. This interpretation is supported by the results on saccade amplitude. Participants made shorter saccades in V condition, fluttering from one position to another. On the opposite, in AV condition, sound could have helped guiding participants' gaze, leading to larger, goal-directed saccade amplitudes. In this study, we compared this impact of sound averaged over entire videos with the one averaged over the 5, 10 and 25 frames following the salient events of video soundtracks. To spot these salient events, we used two auditory saliency models: the Discrete Energy Separation Algorithm and the Energy model. We found that the impact of sound on saccade amplitudes and on the dispersion between eye positions was very similar after DESA peaks, Energy peaks or in general over entire videos. This indicates that the temporal proximity of auditory events spotted by neither the DESA, nor the energy model, increases the effect of sound on eye movements. Moreover, the size of the temporal window over which we averaged eye movement parameters following salient events (5, 10 or 25 frames) did not affect the results. A reason for these results can be that the signal features extracted by the DESA (mean instant Teager-Kaiser energy, amplitude and frequency) might not satisfactorily reflect the way our brain processes auditory information to generate attention. Future studies should investigate more complete auditory saliency models, which use more sophisticated auditory features. For instance, the auditory features used in musical classification (mel-frequency cepstral coefficients (MFCC), delta-MFCC, chromagrams or zero-crossing rates [START_REF] Tzanetakis | Musical Genre Classification of Audio Signals[END_REF]) could be used to successfully integrate sound to visual saliency models. Moreover, one may question the way these features are combined (here, linearly). For a given input, we could estimate the energy of each feature and adjust their weight accordingly. For instance, we could give the MIF a bigger weight when this feature is dominant, like in a whistling sound. Nevertheless, this explanation is not entirely satisfactory. Indeed, although simple, the models we used in this study spotted auditory events that had a perceptual relevance, i.e. that actually were judged as salient by human listeners. To ensure this, we ran a control experiment. Stimuli We chose 14 soundtracks (from 9 to 57 seconds) from the dataset of soundtracks associated to the videos used in the main experiment. These soundtracks were randomly chosen, trying to have exemplar of sounds with music, moving objects, voices, etc. Audio stimuli had the same characteristics as described Section 3.2. For each soundtrack, we computed three sets of peaks: one set using the DESA model, one set using the Energy model and a last set using random peaks. For a given soundtrack, the three sets of peaks had the same number of peaks at least separated by the same minimum interval (one second), like in the main experiment.

Participants and set-up We asked 5 persons to listen to each soundtrack (without video) and to judge whether or not each peak corresponded to a salient audio event. Participants were seated in front of a computer screen with the headphones used for the main experiment. For each soundtrack, three curves with the three sets of peaks (DESA, Energy and Random) were displayed and a blue vertical line marked the progression of the sound. Participant had to tag as salient or not salient 104 peaks x 3 models = 312 peaks. They could replay the audio signals as many time as needed. In average, each participant spent forty-five minutes to fulfill the task. Results For each soundtrack, we obtained a percentage of the peaks identified as salient by the participants for the DESA, Energy and Random models. We ran paired t-test to test which model had the highest percentage of peaks judged as salient. Both DESA and Energy models had a higher percentage than the Random model, with 43% vs. 20% for the DESA model (t(13)=5,46; p < .0001), and 60% vs. 20% for the Energy model (t(13)=5,28; p < .0001). The Energy model had a higher percentage than the DESA (t(13)=-2,8; p < .01). As it could be expected, the peaks spotted by both models were more relevant than the random ones. Moreover, we could notice that when the audio signal contained events that clearly stood out from the background (like speech or noise from a moving object), both DESA and Energy models computed relevant saliency peaks (i.e. events that were judged as salient by a majority of observers). On the contrary, when the input did not contain any particular event (e.g. smooth music, wind blowing), the performance dropped sharply: the peaks emphasized by the models were judged as salient only in few cases. While the relevance of the DESA and Energy models compared to the Random model was expected, we did not expect the better results of the Energy model. However, one should not draw hasty conclusions. The Energy model emphasizes the most evident changes in the audio signal, which are not necessarily the ones that actually draw attention. For instance, a little voice in a noisy environment will not be tagged as salient by the Energy model, although it obviously attracts the auditory attention. Conversely, the DESA model is likely to spot this voice, thanks to its MIF feature. To tag the peaks as salient or not, participants had to listen to each soundtrack several times (at least one time per set of peaks). After two or three listenings, only the most evident changes remain salient, which can explain the preference of the listeners for the Energy peaks. Altogether, both DESA and Energy peaks were globally much more relevant than random ones, which legitimizes the presented analyses.

Conclusions

We have shown that while sound has a global impact on eye movements, this effect is not reinforced just after salient auditory events. This result can be explained if we consider that auditory saliency may entail much more complex and observer-dependent information. The emotions aroused by the soundtrack (e.g. music) or the information contained in it (e.g. speech) can drastically affect our attention on much larger time scale than 5, 10 or even 25 frames. In that case, the temporal proximity of salient events would not be a relevant parameter: the sound would impact on visual exploration in a global way. Altogether, these results indicate that if non-spatial auditory information does impact on eye movements, the exact auditory features capturing observers' attention remain unclear. To successfully integrate sound into visual saliency models, one should investigate the influence of a specific sound on a specific visual feature, and take into account context-sensitive information.

Fig. 2

 2 Fig. 2 Decomposition of a 6840 ms soundtrack (171 frames of 40 ms, upper plot) into energy (second plot), Mean Teager Energy (MTE, third plot), Mean Instant Amplitude (MIA, fourth plot) and Mean Instant Frequency (MIF, fifth plot). The combination of MTE, MIA and MIF gives the auditory saliency curve (lower plot) which is thresholded to spot the saliency peaks (vertical red bars).

Fig. 3

 3 Fig.3Time course of two trials in the visual condition. A fixation cross is presented at the center of the screen, with gaze control. Then, a video sequence is presented in the center, followed by a grey screen. This sequence is repeated for the 50 videos, half without sound (Visual condition), the other half with their original soundtracks (Audio-Visual condition).

Fig. 5 Fig. 6

 56 Fig. 5 Mean saccade amplitude in Audio-Visual an Visual conditions. Saccade amplitudes are averaged over all frames (black) and over the 10 frames following each DESA (blue) peak. Saccades amplitude are given in visual angle (degrees) with error bars corresponding to the standard errors.


Bita Javidfakhr
email: bita_javidfakhr@yahoo.com

Olivier Bellier

Esmaeil Shabanian

Lionel Siame

Laëtitia Léanni

Didier Bourlès

Seiran Ahmadian

Fault kinematics and active tectonics at the southeastern boundary of the eastern Alborz (Abr and Khij fault zones): geodynamic implications for NNE Iran

Keywords: Alborz, active tectonics, Abr Fault, Khij Fault, cosmic ray exposure dating, slip rate

The Alborz is a region of active deformation within the Arabia-Eurasia collision zone.

The Abr and the Khij Faults are two NE-trending left-lateral strike-slip faults in the eastern Alborz that correspond to the Shahrud fault system extended through an area of about 95×55 km 2 . Tectonic landforms typically associated with active strike-slip faults, such as deflected stream channels, offset ridges and fault scarps are documented along the mentioned faults. Detailed analyses of satellite images and digital topographic data accompanied by field surveys allowed us to measure horizontal offsets of about 420 ± 50 m and 400 ± 50 m for the Abr and Khij faults, respectively. A total of 8 quartz-rich samples were sampled and dated from two different fan surfaces using in situ-produced 10 Be cosmogenic dating method.

Minimum exposure ages for the abandonment of the alluvial fan surfaces of 115 ± 14 kyr along the Abr Fault and of 230 ± 16 kyr along the Khij Fault imply that both faults are active with slip rates of about 3-4 mm yr -1 and 1-3 mm yr -1 , respectively. The results of our study provide the first direct quantitative geological estimates of slip rate along these two active A c c e p t e d M a n u s c r i p t 2 faults and place a new constraint on slip distribution between the faults in the eastern Alborz. Fault kinematic studies (from fault slip data) indicate a N35°E-trending maximum stress axis comprising a dominant strike-slip regime in agreement with the geomorphological analyses. The left-lateral strike-slip faulting along the Abr and Khij faults and their associated fault zones in the eastern Alborz can be due to the westward component of motion of the South Caspian Basin with respect to Eurasia and Central Iran.

Introduction

Iran is located within the Alpine-Himalayan orogenic belt in a zone of continental convergence between Eurasia to the North and Arabia to the South which is mostly accommodated by shortening and strike-slip faulting in the mountain belts such as the Great Caucasus, Zagros, Alborz, Kopeh Dagh and also the active Makran subduction zone. Recent geological studies indicate that the Arabia-Eurasia collision in Iran occurred between late Eocene and late Oligocene time (e.g., [START_REF] Allen | Late Cenozoic reorganization of the Arabia-Eurasia collision and the comparison of short-term and long term deformation rates[END_REF][START_REF] Agard | Convergence history across Zagros (Iran): constraints from collisional and earlier deformation[END_REF][START_REF] Vincent | Insights from the Talysh of Azerbaijan into the Paleogene evolution of the South Caspian region[END_REF]2007;[START_REF] Allen | Arabia-Eurasia collision and the forcing of mid-Cenozoic global cooling[END_REF][START_REF] Fakhari | Revised age of proximal deposits in the Zagros foreland basin and implications for Cenozoic evolution of the High Zagros[END_REF][START_REF] Fakhari | Revised age of proximal deposits in the Zagros foreland basin and implications for Cenozoic evolution of the High Zagros[END_REF][START_REF] Boulton | Record of Cenozoic sedimentation from the Amanos Mountains, Southern Turkey: Implications for the inception and evolution of the Arabia-Eurasia continental collision[END_REF]. The Arabia-Eurasia convergence rate is reported to be about 22 mm yr -1 at the longitude of Bahrain south of the Persian Gulf [START_REF] Sella | A model for recent plate velocities from space geodesy[END_REF][START_REF] Mcclusky | GPS constraints on Africa (Nubia) and Arabia plate motions[END_REF]Vernant et al., 2004a;2004b;[START_REF] Reilinger | GPS constraints on continental deformation in the Africa-Arabia-Eurasia continental collision zone and implications for the dynamics of plate interactions[END_REF].

The Alborz accommodates the motion between the South Caspian Basin (SCB) and Central Iran (Figure 1A). The Alborz range has evolved from the Triassic to the present-day during the Cimmeride and Alpine Orogenies [START_REF] Alavi | Tectonostratigraphic synthesis and structural style of the Alborz mountain system in northern Iran[END_REF]. The last major tectonic event that led to widespread deformation in northern Iran and the SCB is coeval with middle Miocene to Recent collision-related compression (e.g., [START_REF] Sengor | Post-collisional tectonics of the Turkish-Iranian plateau and a comparison with Tibet[END_REF]Berberian & Berberian, A c c e p t e d M a n u s c r i p t 3 1981). The south Caspian shorelines move northwest at a rate of about 5 mmyr -1 . The Central Iranian block is defined by internal deformation velocities smaller than 2 mm yr -1 (Vernant et al., 2004b). The Central Iran northward velocities at the longitudes of Damghan and Jajarm have been reported to be of about 9 mm yr -1 and 7 mm yr -1 , respectively [START_REF] Masson | Large-scale velocity field and strain tensor in Iran inferred from GPS A c c e p t e d M a n u s c r i p t 28 measurements: new insight for the present-day deformation pattern within NE Iran[END_REF]. According to [START_REF] Masson | Large-scale velocity field and strain tensor in Iran inferred from GPS A c c e p t e d M a n u s c r i p t 28 measurements: new insight for the present-day deformation pattern within NE Iran[END_REF], at the longitude of Tehran, the Alborz mountain range accommodates a motion rate of about 6 mm yr -1 which is in agreement with the results obtained from regional GPS network in the Alborz (Vernant et al., 2004b). The left-lateral shear across the Alborz is constrained to ~ 4 mm yr -1 (Vernant et al., 2004a). The large amount of geological, geodetic, geomorphological and geophysical data available for the Alborz allows characterizing and qualifying the deformation of this complex zone (e.g., [START_REF] Axen | Exhumation of the westcentral Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics[END_REF]Jackson et al., 2002;Allen et al., 2003a;[START_REF] Masson | Extension in NW Iran inferred from GPS enlightens the behavior of the south Caspian basin[END_REF]2007;Guest et al., 2006a;[START_REF] Guest | Thermal histories from the central Alborz mountains, northern Iran: implications for the spatial and 27[END_REF]2007;[START_REF] Ritz | Active transtension inside central Alborz: A new insight into northern Iran-southern Caspian geodynamics[END_REF][START_REF] Ritz | Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma: Comment[END_REF][START_REF] Tatar | The 2004 May 28 Baladeh earthquake (Mw 6.2) in the Alborz, Iran: Overthrusting the South Caspian Basin margin, partitioning of oblique convergence and the seismic hazard of Tehran[END_REF][START_REF] Ballato | tectonic control on sedimentary facies pattern and sediment accumulation rates in the Miocene foreland basin of the southern Alborz mountains, northern Iran[END_REF][START_REF] Landgraf | Fault-kinematic and geomorphic observations along the North Tehran Thrust and Mosha Fasham Fault, Alborz mountains Iran: implications for fault-system evolution and interaction in a changing tectonic regime[END_REF][START_REF] Nazari | Morphological and paleoseismological analyses of the Taleghan fault, Alborz, Iran[END_REF]Hollingsworth et al., 2010;[START_REF] Solaymani Azad | Left-lateral active deformation along the Mosha-North Tehran fault system (Iran): Morphotectonics and paleoseismological investigations[END_REF]. The N-S convergence of Central Iran together with the SW-ward motion of the SCB toward Central Iran results in a NNE-trending transpressional tectonic regime in the Alborz mountain range. This transpression could have initiated between 3 and 7 m.y. ago [START_REF] Ritz | Active transtension inside central Alborz: A new insight into northern Iran-southern Caspian geodynamics[END_REF].

The Kopeh Dagh mountain range includes Mesozoic and Tertiary sediments, which were folded during the Oligo-Miocene orogenic movements [START_REF] Stocklin | Structural history and tectonics of Iran: a review[END_REF][START_REF] Stocklin | Mesozoic-Cenozoic Orogenic Belts; Data for Orogenic Studies; Alpine-Himalayan Orogens[END_REF][START_REF] Harb | The stratigraphy, tectonics and petroleum geology of the Kopet Dagh region, northeastern Iran[END_REF][START_REF] Lyberis | Oblique to orthogonal convergence across the Turan block in the post-Miocene[END_REF]. The right-lateral Main Kopeh Dagh Fault system (MKDF) is characterized as the northwestern boundary of the Kopeh Dagh range (Figure 1) and considered as a seismically active structure (Trifonov, 1978). Previous studies [START_REF] Regard | Cumulative right-lateral fault slip rate across the Zagros -Makran transfer zone and role of the Minab-Zendan fault system within the convergence accommodation between Arabia and Eurasia (SE Iran)[END_REF][START_REF] Authemayou | Late Cenozoic partitioning of oblique plate convergence in the Zagros fold-and-thrust belt (Iran)[END_REF]2009;[START_REF] Dortz | Holocene right-slip rate determined by cosmogenic and OSL dating on the Anar fault, Central Iran[END_REF]Shabanian et al., 2009a;2009b) proposed estimates of long-term slip rates for some of the major active fault systems in Iran.

A total slip rate of 9 ± 2 mm yr -1 is proposed for strike-slip faulting in the Kopeh Dagh, A c c e p t e d M a n u s c r i p t 4 resolving for western Kopeh Dagh as an average northward and westward slip rates of about 8 and 4 mm yr -1 , respectively (Shabanian et al., 2009a).

The South Caspian Basin behaves as a relatively rigid aseismic block [START_REF] Priestley | Implications of earthquake focal mechanism data for the active tectonics of the south Caspian Basin and surrounding regions[END_REF] and is suggested to be a trapped oceanic remnant [START_REF] Berberian | The southern Caspian: a compressional depression floored by a trapped, modified oceanic crust[END_REF]. This implies that uplift of the high Alborz and Talesh mountains loaded the SCB, causing accumulation of thick late Cenozoic deposits [START_REF] Axen | Exhumation of the westcentral Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics[END_REF]Allen et al., 2003b). The SCB is surrounded by orogens including the Great Caucasus, Alborz, Kopeh Dagh, and the Great Balkhan. Based on the motion of a single GPS site on the Caspian shoreline, the SCB moves northwest with respect to Eurasia at a rate of 6 ± 2 mm yr -1 and the motion across the central Alborz is suggested to be roughly 5 ± 2 mm yr -1 of N-trending shortening and 4 ± 2 mm yr -1 of leftlateral strike-slip (Vernant et al., 2004b). A higher estimate of 11 ± 2 mm yr -1 has been proposed for the South Caspian-Eurasia motion by [START_REF] Copley | Active tectonics of the Turkish-Iranian plateau[END_REF]. [START_REF] Ritz | Active transtension inside central Alborz: A new insight into northern Iran-southern Caspian geodynamics[END_REF] mentioned Pleistocene as the age of the SCB northward motion towards Eurasia and suggest that this may have caused not only the change from a compressional tectonic regime to a transpressional one but also the transtension in the internal parts of the range. They emphasize that the internal domain of central Alborz is affected by an active transtensional regime with a WNW-trending extensional axis.

In the current study we present a morphotectonic analysis along two NE-trending leftlateral strike-slip faults, i.e., the Abr and Khij faults (Figure 2) which belong to the Shahrud fault system affecting the eastern Alborz. The determination of the kinematics of the active deformation along the Shahrud fault system using fault slip vectors measured in the field combined with the determination of the abandonment age of two alluvial fan surfaces offset along the faults using in situ-produced cosmogenic 10 Be exposure dating of quartz pebbles allow us to establish the rate and kinematics of the late Quaternary deformation in the southeastern limit of the eastern Alborz. At the regional scale, tectonic implications of the

Seismo-tectonic setting and structural framework

Tectonics

The Alborz is located south of the SCB consisting late Precambrian to Quaternary deposits (Figure 1A). Tectonic evolution of the Alborz range results from a combination of thrust activity within the orogen and, based on the chronology of the foreland basin deposits [START_REF] Ballato | tectonic control on sedimentary facies pattern and sediment accumulation rates in the Miocene foreland basin of the southern Alborz mountains, northern Iran[END_REF], of frontal accretion. There are different ideas about the geometry and kinematics of faulting in the Alborz range (e.g., Jackson et al., 2002;Allen et al., 2003a;Guest et al., 2006a;[START_REF] Ritz | Active transtension inside central Alborz: A new insight into northern Iran-southern Caspian geodynamics[END_REF][START_REF] Zanchi | Inversion tectonics in central Alborz, Iran[END_REF][START_REF] Yassaghi | Influence of a transverse basement fault on along-strike variations in the geometry of an inverted normal fault: Case study of the Mosha Fault, Central Alborz Range, Iran[END_REF]Landgarf et al., 2009). For instance, [START_REF] Ritz | Active transtension inside central Alborz: A new insight into northern Iran-southern Caspian geodynamics[END_REF] indicated that the margins of Alborz are affected by a transpressional regime, while there is a general left-lateral shear inside the central Alborz range. They have interpreted the total kinematic view of the Alborz as a general strike-slip regime with a change in the position of σ 1 from a horizontally NNE-trending to a vertical one between the margins and the internal domain.

The compressive deformation has been active since Oligocene (Allen et al., 2003a;2003b). There are active faults along the southern part of the mountain belt and also along the Caspian coast [START_REF] Bachmanov | Active faults in the Zagros and central Iran[END_REF]. The present-day tectonic framework of the range is characterized by a NE-SW compression causing a lateral inversion of the sense of motion from right to left-lateral (Allen et al., 2003a) 1). In eastern part of the Alborz range, the Gorgan Schists are thrusted against Quaternary deposits by the Khazar Fault.

Seismicity

A global record of strain distribution over the last century is available by instrumentally recorded earthquakes, but the interval times between seismic events on individual faults can be up to several thousands of years. Following, we summarize the most important historical and instrumental earthquakes in the eastern Alborz.

A relatively complete catalogue of the historical earthquakes in north-eastern Iran is available (Tchalenko, 1975;[START_REF] Ambraseys | A history of Persian earthquakes[END_REF][START_REF] Berberian | Patterns of historical earthquake rupture in the Iranian Plateau[END_REF]2001).

The largest earthquakes in eastern Alborz are reported in 856, 1825, 1890, 1953 and 1985 with magnitudes ranging from 6.2 to 7.9 (based on the macroseismic data). 200,000 persons were killed in the catastrophic earthquake of 856. According to a recent paleoseismology study in eastern Alborz (Hollingsworth et al., 2010), the Astaneh Fault's rupture is consistent with rupture during the historical 856 A.D. Qumis [START_REF] Ambraseys | A history of Persian earthquakes[END_REF] earthquake in NE Iran. Historical earthquakes are also reported in 874, 1436, 1470 and 1498 in the area of Gorgan and Gonbad-Kavous. The destructive earthquakes of 1890 in the Shahrud region could be concerned with the active faults of this study on the basis of the reported regions of maximum destruction [START_REF] Ambraseys | A history of Persian earthquakes[END_REF]. The earthquake of 1944 caused major damage in Gorgan [START_REF] Ambraseys | A history of Persian earthquakes[END_REF]. A left-lateral faulting has been involved in the Karnaveh earthquake of 1970 (Mw 6.4), based on the analysis of its aftershock zone (Jackson & Fitch, 1979). The earthquake of 1985 occurred in the northern part of the Abr Fault (Table 1, Figure 1B).

According to morphotectonic and paleoseismologic studies (Hollingsworth et al., 2010) frequent stream offsets along the trace of the Astaneh Fault (eastern Alborz) may record the A c c e p t e d M a n u s c r i p t 7 cumulative offset of the last two to five earthquakes in Holocene. We emphasize that eastern Alborz is more seismically active than western Alborz. Few earthquakes occurred in the SCB and it is thus considered seismically relatively quiet although it is surrounded by very active fault zones (e.g., [START_REF] Priestley | Implications of earthquake focal mechanism data for the active tectonics of the south Caspian Basin and surrounding regions[END_REF].

Shahrud Fault System

The 400-km-long Shahrud fault system is characterized by active NE-trending left-lateral strike-slip fault zones (Astaneh, Shahrud, Abr and Jajarm fault zones) extending at the SE flank of the eastern Alborz. The combined geomorphic characteristics suggest coeval strikeslip and reverse faulting mechanism for the ongoing activity of this fault system, characterizing a transpressional tectonic regime that is confirmed by the kinematic inversion results (Javidfakhr, 2010).

The left-lateral Astaneh fault zone (the westernmost part of Shahrud fault system) cuts across the topography north of Damghan city. This NE-trending fault zone runs across the southern boundary of eastern Alborz for over 150 km (Hollingsworth et al., 2008;[START_REF] Dunaï | Cosmogenic Nuclides. Principles, Concepts and Applications in the Earth Surface Sciences[END_REF].

Western part of Shahrud fault system includes the Shahrud fault zone defined by two active faults, the 130-km-long Shahrud Fault and the 70-km-long Tazareh Fault (Figure 2). The geomorphic expression of the Shahrud Fault is characterized by abrupt fronts of the mountain range. The high-angle Shahrud Fault is a combination of the E-and NE-trending fault segments running along the eastern Alborz Mountains.

Eastern part of the Shahrud fault system consists of multiple fault zones: (1) the Jajarm fault zone which is an extended fault zone in this broad fault system, (2) the Cheshmeh-Nik fault zone which, at a first glance, seems to be the continuation of the Abr fault (Figure 2). However, this is not certain given the gap visible on satellite images of about 1 km between them at the eastern termination of the Abr Fault. the southern part of Shahrud fault system can A c c e p t e d M a n u s c r i p t 8 be described as the northern margin of Central Iran. The Jajarm Fault is a left-lateral strikeslip fault located in the north of Jajarm city running for about 150 km in the NE direction (e.g., Hollingsworth et al., 2008;Javidfakhr, 2010). The active Jajarm Fault passes through Quaternary and Plio-Quaternary deposits in most of its trace. The Jajarm Fault is parallel to another left-lateral fault segment along its north side, i.e., Cheshmeh-Nik Fault (Figure 2).

The area between the Jajarm and Cheshmeh-Nik faults is covered by Neogene continental red beds and Jurassic limestones as well as scattered Quaternary deposits. The Abr and Khij faults are two active boundary faults, located in the central part of the Shahrud fault system, that have separated the mountainous Alborz region from Central Iran.

The Abr fault zone

The geomorphic expression of the fault zone in the context of the obtained fault kinematic data suggests a reverse component of faulting in the Abr fault zone. The Abr Fault extends for about 95 km in the eastern Alborz and runs northeast between Nekarman and Tilabad villages (Figures 2B,3A). The NE-trending Abr Fault is a left-lateral strike-slip fault with a reverse component of dip-slip that marks a distinct topographic boundary between mountainous regions and Quaternary basins (Figure 3E), displaying morphological evidence of active faulting (Figures 3C and3D). The deflection of rivers across this segment of the Abr Fault suggests this segment has been active in the Quaternary (Figure 3C).

The Abr Fault is parallel to the Khij Fault in most parts of its length, with a spacing of ~20 km between them. The trend of the Abr Fault changes in its 40 km long eastern segment and turns to the East. In the termination zone of the Khij Fault, near Tilabad village, the spacing between the two faults decreased to 2 km due to a change in the orientation of the Abr fault trace (Figure 3A).

A c c e p t e d M a n u s c r i p t 9

There are no geometric and/or structural changes at fault terminations. At the termination zone, relative fault displacement reduces to zero and is transferred to other structures, laterally or vertically, away from the fault end-point [START_REF] Legg | Termination of strike-slip faults at convergence zones within continental transform boundaries: examples from the California Continental Borderland[END_REF]. The process of fault termination depends on scale since faults interact with various features of different length scales and since also the length scale of a fault increases during fault growth [START_REF] Alessio | Fault terminations and barriers to fault growth[END_REF]. The trend of the Abr Fault changes noticeably in a relatively wide area of its western termination zone (Figure 3B). The Quaternary trace of the Abr Fault terminates in a cluster of distributed small faults within a horsetail structure arrangement (Figure 3B).

There is a 12-km-long active reverse fault in the south of Nekarman village parallel to the trace of the Abr Fault (Figure 4A). The outcrop of this fault was limited to few sites. Vertical offset was estimated from the profile perpendicular to the fault trace which is shown in Figure 4B. The vertical offset of this fault is estimated at about 24 m (Figure 4C) using the 1:20,000 topographic map.

The Khij fault zone

The left-lateral Khij Fault passes through Khij village (Figure 3A), and strikes NE, parallel to the eastern Alborz range for about 55 km. The trend of the Khij Fault remains constant all along its trace. At its both ends, the trace of the fault remains sharp. There are some nearly parallel small faults (10-15 km) in the central and western part of the fault. These faults offset the associated terraces left-laterally for several tens of meters (Figure 3D). They probably should be responsible for a negligible part of left-lateral slip across the fault zone. Some channels in the western part of the Khij Fault are deflected with left-lateral offsets of about 20-100 m (Figure 3D). Vertical deformations such as small scarps with reverse faulting arise along the Khij Fault, but most of the features are locally observed and result from strikeslip faulting. The kinematic measurements were numerically processed to analyze the fault population data [START_REF] Carey | Analyse théorique et numérique d'un modèle mécanique élémentaire appliqué à l'étude d'une population de failles[END_REF][START_REF] Carey | Recherche des directions principales de contraintes associées au jeu d'une population de failles[END_REF], a mean best-fitted deviatoric stress tensor for a fault population being calculated through the minimization of the angular deviation between the measured striation and the resolved shear stress. This inversion method assumes that the slip established by the striation is in the same direction as the resolved shear. The orientation of the principal stress axes (σ 1 , σ 2 , σ 3 ) and the stress ellipsoid shape parameter (R), defined as R = (σ 2 -σ 1 )/(σ 3 -σ 1 ), are the results thus obtained through the applied inversion method [START_REF] Carey | Analyse théorique et numérique d'un modèle mécanique élémentaire appliqué à l'étude d'une population de failles[END_REF][START_REF] Mercier | Paleostress determinations from fault kinematics: application to the neotectonics of the Himalayan-Tibet and the central Andes[END_REF][START_REF] Bellier | Recent state of stress change in the Walker Lane zone, western Basin and Range province, United States[END_REF].

Relative chronology of the striations and their coherence with the regional tectonic events are considered to separate the distinct families of striation. We detected two slip generations of striations revealed by clear crosscutting striations on fault planes in most of the measurement sites. Just one slip direction on a given fault plane can be produced by a distinct stress deviator (σ 1 , σ 2 , σ 3 and R) due to the assumptions of the fault kinematic inversion methods. Three successive stress states are distinguished through analysis of distinct datasets after the separation of the slip generations (Javidfakhr, 2010).

Modern state of stress deduced from the fault kinematic analysis

The modern state of stress was analyzed using fault kinematics indicated by the last generation of striation sets measured mostly in Quaternary deposits (Table 2). Table 2 consists of the inversion results of the fault slip data accompanied by the lithology definition of different sites. It shows that R values (that must lie between 0 and 1) are in the range 0.63 to 0.98. All the tensors representative of the modern stress state indicate the same direction of It is nevertheless possible to distinguish two specified regimes in the studied region. 9 out of 11 sites indeed show strike-slip regimes (with a minor reverse slip component) while two sites (Figure 6, sites number 1 and 8B) indicate mostly compressional stress regime. The inversion results obtained from five measurement sites along the Abr Fault trace characterized a relatively homogenous transpressional stress regime (Figure 5). At site number 8, two diagrams of inversion solution with σ 1 trending N66°E and N12°E for the diagram 8A and 8B, respectively (Figure 6), are obtained. However, there is a minor fault almost parallel with the Khij Fault in the vicinity of this number 8 site accompanied by an older fault (almost perpendicular to the trend of Khij Fault) cut by the Khij Fault. This intersection of faults may be the reason of this local variation. Structural framework of the region is thus simply explained by strike-slip and compressional stress deviators (Figure 6). Considering the fault geometry, the dominant left-lateral strike-slip faulting along the Abr and Khij faults is in agreement with an average computed stress deviator, N30°E trending horizontal σ 1.

Geomorphic investigations

Methodology: mapping and site selection

The detailed structure of strike-slip fault zones and their evolution are basic subjects in understanding of the ongoing deformation in this region. Major fault segments have been mapped in order to recognize the structural and geomorphic characteristics of the fault

Offsets

Offset of the Abr alluvial fan

The geomorphic expression along the Abr Fault is characterized by left-laterally offset features such as alluvial fans, stream channels and shutter ridges. Figure 7A shows the morphology of the fan system chosen because it is well preserved from surface outwash and fluvial erosion. In addition to a feeder channel clearly left-laterally offset by the fault, their distinct geomorphology and elevations allow distinguishing five main surfaces. From younger to older, they are classified as Q1 to Q5 (Figure 7B). The geomorphic relationship clearly indicates that the feeder channel was emplaced after the abandonment of the Q3 unit.

Figure 7C shows the geomorphic map of the Abr fan reconstructed to measure the cumulative offsets. With ~700 m of back-slip, the alignment between the fan's feeder channel The amount of left-lateral offset ranges from ~60 m (Figure 7D) for the younger drainages incised in alluvial fans located in the west of the site area to ~700 m at the dating site (Figure 7C). It seems that the most likely amount of left-lateral offset at this site is ~420 m considering the active stream deflection (~280 m) shown in Figure 7E. However, subparallel faults locating in the area may accommodate additional left-lateral motion across the fault zone.

Offset of the Khij alluvial fan

The Khij Fault clearly cuts across the Quaternary surfaces incised by abundant streams in an extended area comprising the dating site (Figure 8A). The strike-slip component dominates the reverse component along the fault (Figures 8b,9). Along its straight trace, the fault left-laterally offsets the Quaternary landforms such as fan surfaces and river channels (Figure 9). The restoration of different fan surfaces in the Khij site is difficult due to effects of erosion in the area. Figure 9 illustrates our reconstruction of offsets across the fault zone using the high resolution Quickbird image exhibiting the obvious left-lateral displacement of streams. The actual offset is about 400 m because the trend of the active drainages is relatively similar in both the upstream and downstream segments (Figure 9). In this area, it is difficult to distinguish the original fan boundaries. We suggest that the uncertainty in offset across the Khij Fault is no more than 50 m because of the high similarity in trend of the offset features north and south of the fault zone. However, these uncertainties have been considered

A c c e p t e d M a n u s c r i p t 14 in establishing bounds on the value of offset. Finally, we suggest 400 ± 50 m of back-slip to restore the offset, which is the best estimate deduced from the reconstruction of three major offset markers in the proximity of the dating site (Figures 9B,9C). The displacement of the drainage system is surely younger than the displacement of the fan surfaces. Because the streams incised in the fan surfaces are younger than the fan abandonment, the cumulative offset value of 400 ± 50 m is considered as the minimum horizontal offset possible for this segment of Khij Fault. Moreover, digital topography maps with the scale of 1:20,000 were used to determine the vertical displacement across the Khij Fault. Vertical offsets were estimated from the profiles perpendicular to the fault trace after restoring the coeval lateral offset (Figure 8B). The dating site and its surrounding alluvial fans are shown in Figure 8A.

The vertical displacement for this segment of the Khij Fault is about ~12 m (Figure 8B) which represent the prominent strike-slip character of the Khij Fault. The Quaternary surface is gently tilted over Neogene deposits in a site located in the vicinity of Khij dating site (Figure 10).

Cosmogenic dating and fault slip rates

Since the progress made over the last decades by the Accelerator Mass Spectrometry (AMS) technique, cosmogenic nuclides are extensively used to quantify the processes affecting and shaping the Earth's surface (see review in Gosse and Philips, 2001;[START_REF] Dunaï | Cosmogenic Nuclides. Principles, Concepts and Applications in the Earth Surface Sciences[END_REF]. In particular, absolute dating of landforms for deciphering climatic history or tectonic history has been more specifically developed and the concepts and principles of the Cosmic Ray Exposure (CRE) dating method relatively well established. Due to its half life of 1.39 ± 0.01My (Korchinek et al., 2009;[START_REF] Chmeleff | Determination of the 10 Be half-life by multicollector ICP-MS and liquid scintillation counting[END_REF] and because its production is well known in quartz mineral, in situ-produced 10 Be is an adequate candidate to date surface A c c e p t e d M a n u s c r i p t 15 samples to 2 My in best conditions [START_REF] Guralnik | 10 Be exposure ages of ancient desert pavements reveal Quaternary evolution of the Dead Sea drainage basin and rift margin tilting[END_REF] or more frequently from several ky up to 1 My depending of the denudation rate affecting the samples.

Samples were prepared for AMS 10 Be measurements following chemical procedures adapted from [START_REF] Brown | Examination of surface exposure ages of Antarctic moraines using in situ-produced 10 Be and 26 Al[END_REF] and [START_REF] Merchel | An update on radiochemical separation techniques for the determination of long-lived radionuclides via accelerator mass spectrometry[END_REF]. Samples were first crushed and sieved. Pure quartz was obtained from the 250-500 µm fraction by repeated H 2 SiF 6 -HCl etching; atmospheric 10 Be was subsequently eliminated by sequential dissolutions with diluted HF. Prior to complete HF digestion of silica, a weighted 100 µl of a 3025 ppm home-made carrier solution was added to allow for 10 Be/ 9 Be ratio calculation after AMS measurement.

The spiked solutions were finally purified by solvent extractions of Be acetylacetonate in presence of EDTA followed by precipitations of Be(OH) 2 at pH 8.5 and rinsing. The final precipitate, dissolved in a few drops of HNO 3 , is dried and heated at 900°C to obtain BeO.

Beryllium oxide was mixed to 325 mesh niobium powder prior to measurement at ASTER, the French AMS national facility located at CEREGE, Aix en Provence. All 10 Be concentrations are normalized to 10 Be/ 9 Be SRM 4325 NIST reference material with an assigned value of (2.79 ± 0.03).10 -11 . This standardization is equivalent to 07KNSTD within rounding error. The 10 Be half-life of (1.39 ± 0.01) × 10 6 years used is that recently recommended by Korschinek et al. (2009) and [START_REF] Chmeleff | Determination of the 10 Be half-life by multicollector ICP-MS and liquid scintillation counting[END_REF] according to their two independent measurements. Analytical uncertainties (reported as 1) include a conservative 0.5% external uncertainty based on long-term measurements of standards, a one sigma statistical error on counted 10 Be events, and the uncertainty associated with the chemical blanks correction. Two chemical blanks prepared with the samples yield similar 10 Be/ 9 Be ratio of (3.3 ± 0.8) × 10 -15 and (3.4 ± 0.7) × 10 -15 , 2 orders of magnitude smaller than the minimum measured ratio. An additional 6% production rate uncertainty is considered for CRE age calculation [START_REF] Stone | Air pressure and cosmogenic isotope production[END_REF].

A c c e p t e d M a n u s c r i p t 16 A modern 10 Be production rate at sea-level and high-latitude of 4.5 ± 0.3 atoms/g/yr, computed for internal consistency from the data of [START_REF] Stone | Air pressure and cosmogenic isotope production[END_REF] according to the conclusions of the recently published study on absolute calibration of 10 Be AMS standards by [START_REF] Nishiizumi | Absolute calibration of 10 Be AMS standards[END_REF], was used. This sea-level and high-latitude production rate has then been scaled for the sampling altitudes and latitudes using the scaling factors proposed by [START_REF] Stone | Air pressure and cosmogenic isotope production[END_REF] because, using the atmospheric pressure as a function of altitude, they take into account the physical properties of cosmic ray particle propagation in the atmosphere and include an improved account for the muonic component in the total cosmogenic production. Corrections for local slope and topographic shielding due to surrounding morphologies following [START_REF] Dunne | Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depthon sloped surfaces[END_REF] appear negligible for both sampling sites.

Erosion is a major source of uncertainty in the surface exposure dating. Considering the very low erosion rate of 0.002 mm yr -1 estimated for the alluvial fan surfaces of Central Iran using 10 Be and 36 Cl cosmogenic nuclides (Le [START_REF] Dortz | Cinématique de l'Iran central et oriental: Morphotectonique et datations cosmogéniques et OSL[END_REF] and the geomorphic freshness of the Abr and Khij alluvial fans, erosion was reasonably considered as negligible throughout this work. This implies that the CRE ages calculated from the measured 10 Be concentrations are minimum exposure ages.

The 10 Be concentration in the sampled surface quartz boulders being directly related to alluvial fan abandonment, alluvial fans that have been preserved during left-lateral displacements along the studied faults have been dated in order to estimate their long-term slip rates. Long-term slip rates on strike-slip faults can indeed be obtained by dividing horizontal geomorphic offset by the CRE age of the surface considering the uncertainties associated to the different applied methods.

To minimize the possibility of rotation or rolling, boulders embedded into the ground surface with top surfaces as flat as possible were selected. Even if 6 boulders were collected on each fan, only 3 and 5 samples at the Abr and Khij sites, respectively, provided data 3.

Exposure ages and the slip rate of the Abr Fault

Along the Abr Fault, in situ-produced 10 Be concentration have been measured in samples collected from the Q3 fan surface which has been offset by 420 ± 50 m (Figure 7B).

Three dated samples yield minimum exposure ages of 96 ± 6 kyr, 118 ± 8 kyr and 131 ± 9 kyr (Table 3). The observed low dispersion of the Q3 surface ages may result from different rates of exhumation to the surface of sampled boulders that were previously buried within the alluvial deposits. If this is the case, the oldest CRE age yields the more reliable fan surface abandonment age. If this low dispersion results from different erosion rates affecting the surface sampled boulders, the more 10 Be concentrated sample yielding the oldest CRE age is the less affected by erosion and thus the sample which again yields the more reliable fan surface abandonment age. Considering then the upper bound of the CRE age series as the more reliable fan surface abandonment age and the horizontal offset of 420 ± 50 m recorded by this surface yields a minimum slip rate of 3.2 ± 0.5 mm yr -1 along the Abr Fault.

Exposure ages and the slip rate of the Khij Fault

The CRE ages calculated from the in situ-produced 10 Be concentrations measured in the five quartz-rich pebbles embedded in the fan surface at the Khij site are generally older than those calculated from the Abr site samples. They indeed range from 152 ± 10 to 364 ± 24 kyr.

More precisely, three samples, statistically belonging to the same population, cluster around a weighted-mean CRE age of 165 ± 6 kyr while the two other samples are significantly older (Table 3). The cumulative offset of the fan surface is most likely between 350 and 450 m.

A c c e p t e d M a n u s c r i p t 18

There is two ways to interpret this observed distribution. Either, as discussed above, erosion can reasonably be considered as negligible and the two older ages result from inherited 10 Be acquired during a previous exposure episode in the area. In that case, the weighted-mean 10 Be minimum CRE age of 165 ± 6 kyr is the best estimate for the fan surface abandonment age (Figure 11). Or, the negligible erosion rate assumption is not valid, and the oldest CRE age of 364 ± 24 kyr yields the more reliable fan surface abandonment age. In this case, the younger ages result from different erosion processes affecting the fan surface.

Considering both interpretations, the cumulative 400 ± 50 m horizontal offset recorded by the Khij Fault constrains the Quaternary left-lateral slip rate on this segment of the fault to range between 2.4 ± 0.3 mm yr -1 to 1 ± 0.2 mm yr -1 .

The dated fan has been tectonically uplifted ~12 m based on quantitative analysis of the slope profile (Figure 8). Taking into account the errors involved in the mean vertical offset measurement and the uncertainties associated to the CRE ages, the vertical slip rate along this segment of the Khij Fault is calculated to be less than 0.07 mm yr -1 .

Discussion and conclusion

Active deformation in the northeastern boundary of the Arabia-Eurasia collision zone is taken up by strike-slip faults, which is oblique to the convergence direction in this boundary zone (Vernant et al., 2004b;[START_REF] Reilinger | GPS constraints on continental deformation in the Africa-Arabia-Eurasia continental collision zone and implications for the dynamics of plate interactions[END_REF]. The whole compression in Alborz mountain belt seems to be roughly orientated north-south with a rate of about 8 ± 2 mm yr -1 and the shortening rate absorbed by the Alborz and the SCB is about 14 ± 2 mm yr -1 (Vernant et al., 2004a).

It is essential to distinguish the present-day tectonic configurations in active deformation domains. This paper focused on a major fault system at the SE boundary of eastern Alborz

The major strike-slip faults are mainly located within the internal parts of the Alborz range A c c e p t e d M a n u s c r i p t 19 (e.g., Allen et al., 2003a). Active left-lateral strike-slip faults trend ENE and WNW in the eastern (Allen et al., 2003a;[START_REF] Ritz | Active transtension inside central Alborz: A new insight into northern Iran-southern Caspian geodynamics[END_REF] and western Alborz [START_REF] Axen | Exhumation of the westcentral Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics[END_REF]Guest et al., 2006a;2007), respectively. It is noticeable that there are much more evidences of leftlateral strike-slip faulting in the eastern Alborz (e.g., Hollingsworth et al., 2010) than in the west. Allen et al. (2003a) Shabanian et al., 2009b). This mechanism is due to the oblique convergence of this boundary zone. The Abr and Khij left-lateral strike-slip faults and their associated faults in the Shahrud fault system are involved in the northward convergence of Central Iran and the north-westwards motion of the SCB relative to Eurasia.

We concentrated on the analysis of geomorphic features to enrich present-day tectonic activity of the region. Alluvial fans can record cumulative displacements in tectonically active regions. Accurate dating of geomorphic offsets can lead us for a quantitative interpretation of long term slip rates on major active faults. It also helps to have a better understanding of the regional tectonics and to establish the deformation history of the area. We have presented evidence of late Quaternary activity along the Abr and Khij fault zones in eastern Alborz. Slip rates are calculated for the strike-slip Abr and Khij faults taking into account the geomorphic evidence and the applied 10 Be exposure dating method. A long term left-lateral slip rate of 3.2 ± 0.5 mm yr -1 along the NE-trending Abr Fault has an important implication to our understanding of the deformation in the eastern Alborz. Along the Khij Fault, the horizontal slip rate ranges between 2.4 ± 0.3 mm yr -1 and 1 ± 0.2 mm yr -1 . The vertical slip rate of 0.07 mm yr -1 is much slower than the strike-slip rate of the Khij Fault, indicating that strike-slip is the dominant mechanism of the late Quaternary activity of this fault which is also supported by our geomorphic and fault kinematic data. The absence of clear vertical scarps in the fan deposits is also representative for the dominant component of strike-slip.

In the study area, the active deformation north and east of Bastam city is principally taken up by the Abr and Khij left-lateral strike-slip faults (Figure 2). The Abr fault zone has generated three earthquakes (Mw 4.9-Mw 6) in the past thirty years (Table 1). However, moderate earthquakes could be judged as signals for considerable hazard if they are frequently reported in populated regions. It is fruitful to analyze the earthquake focal mechanisms considering the structural setting of the fault slip data in an extended region to establish more A c c e p t e d M a n u s c r i p t 21 precise seismotectonic framework of this boundary zone. The regional stress state (N47°E) is analyzed using the inversion of earthquake focal mechanisms in a vast area between central Alborz and western Kopeh Dagh in 52°-56°E (Javidfakhr, 2010). This result is in close agreement with the modern stress state deduced from inversion of fault kinematic data. The modern stress field is characterized by a regional NE-trending horizontal σ 1 (Figure 5) based on kinematic inversion results. All the earthquake focal mechanisms are not compatible with this regional stress state. However, we suggest that the complex dynamic characteristics of the study region enclosed between two deformation systems of Alborz and Kopeh Dagh seem to be the most effective factor to characterize the changes in the attitude of σ 1 axis.

Our results can be compared with GPS data that estimate a left-lateral strike-slip motion rate ranging between 2 and 6 mm yr -1 along the faults in the Alborz (Vernant et al., 2004), to have a better assess to the validity of the results within a regional active deformation framework. It should be noticed that the rate estimate provided by the GPS data, integrates strain over a much larger area.

Detailed geomorphic studies complemented by structural analysis in other parts of the Shahrud fault system will characterize a complete scheme for the geodynamic evolution of this tectonically active zone. [START_REF] Reilinger | GPS constraints on continental deformation in the Africa-Arabia-Eurasia continental collision zone and implications for the dynamics of plate interactions[END_REF] [START_REF] Carey | Recherche des directions principales de contraintes associées au jeu d'une population de failles[END_REF]. The labels refer to the site locations presented in Figure 5. The large arrows indicate the azimuth of σ 1 . A fixed solution [START_REF] Bellier | Recent state of stress change in the Walker Lane zone, western Basin and Range province, United States[END_REF] was applied to deal with the sites comprising less than four well-distributed fault directions (site number 6). One of the principal stress axes is fixed to remain purely vertical in the fixed inversion method. Measurement sites comprising two tectonic regimes in each state of stress are presented by adding "A" suffix to the identifying number of the site. The suffix "B" is added to the measurement site number in the cases that the data were not enough and/or well-distributed to be reliable as an inversion solution. Note that the trend of σ1 is similar in site number 1 comprising the limestone of Cretaceous age with other sites measured in Quaternary or Neogene deposits (refer to Table 2).

  deduced results presented in this paper provide new geological constraints on active tectonics of the Alborz Mountains.

  as well as the activation of ESE-trending thrust faults. Late Cenozoic tectonics in the eastern Alborz is characterized by oblique and strikeslip faulting which reactivated inherited Mesozoic structures. The active tectonic framework of the eastern Alborz consists of major NE-trending left-lateral faults. One of the main structures in northern margin of the Alborz range is the Khazar Fault extended in western partA c c e p t e d M a n u s c r i p t 6 of our study area (Figure

  A c c e p t e d M a n u s c r i p t 11 σ 1 stress axes despite different formation ages. For example, the trend of σ 1 at site number 1 comprising Cretaceous age limestone is similar to that at other sites measured in Quaternary or Neogene deposits. Figure5indicates the location of the measurement sites and the state of stress deduced from fault kinematics inversion. According to the inversion results for a major part of sites, the modern stress state is characterized by a NE-trending σ 1 ranging from N14°E to N46°E and indicates a relatively homogeneous stress field.

  A c c e p t e d M a n u s c r i p t 12 system. Different types of satellite images such as LANDSAT ETM+ (14 m resolution), SPOT5 images (5 m resolution) and SRTM data were used to improve the visualization of the traces of the fault systems and better constrain their geometry considering the structural linkage between the different fault segments. The observation of the recent deposits (Pleistocene until present time) surface morphology allows quantifying the cumulative tectonic displacement which occurred during the last million years. In the current study, sites indicative of distinct fault activity were selected to estimate the fault slip rates. Firstly, the morphology of each site was precisely mapped by combining field observations with evidences from SPOT5 images in order to determine horizontal offsets through examination of the best match possible for the trend of streams incising Quaternary deposits. Study of the segments along the Abr and Khij faults offered evidences for late Quaternary geomorphic features associated with strike-slip faulting, such as stream offsets including tail-cut and head-cut streams and alluvial fan offsets.

  axis is restored. The fan's axial channel is beheaded and has a left-laterally offset with respect to its feeder channel. This reconstruction assumes that no rotation occurred. However, some rotation north of the fault zone would only increase the offset by about 20-30 m.

  samples failed through chemical processing due to their unsuitable lithology for 10 Be cosmogenic dating. The measured 10 Be concentrations and the calculated minimum CRE ages are presented with their associated uncertainties in Table

  proposed a model for late Cenozoic structural evolution of the Alborz. They suggested that the continued compressional deformation (probably initiated in Miocene) is accompanied by left-lateral strike-slip faulting along the length of the Alborz range during Plio-Quaternary accommodating the westward motion of the SCB basement with respect to Central Iran. The ongoing deformation in the Tehran's piedmont (central Alborz) is accommodated along N070°E to N110°E trending left-lateral strike-slip faults and low angle dip thrusts (Abbassi & Farbod, 2009). Ritz et al. (2008) estimated a total left-lateral displacement along the active strike-slip faults in central Alborz of about 3-6 km and a cumulative left-lateral displacement of about 3 km along the central Mosha Fault (central Alborz) is estimated by Solaymani Azad et al. (2011). Based on their recent morphotectonic analysis within central Alborz (Solaymani Azad et al., 2011), the Mosha and North Tehran faults are active leftlateral strike-slip faults displaying a left-stepping en-echelon pattern. Hollingsworth et al., (2010) suggested that frequent stream offsets of 15-20 m along the active trace of the Astaneh Fault could record the cumulative offset of the last two to five earthquakes during Holocene. The Abr and Khij faults strike parallel to the structural trend of the Alborz range. The trace of the Abr Fault corresponds to the contact between pre-Neogene rock units in north and Quaternary deposits in south. The major neighboring structures such as the Khazar, Shahrud and Jajarm fault zones can significantly influence the fault zone behavior and motion rate in the eastern Alborz. However, the left-lateral motion of the Shahrud fault system and the rightlateral one on the Main Kopeh Dagh Fault are simple explanations for the westward motion of

  toward the SCB (e.g.,

Figure 1 .

 1 Figure captions: Figure 1. A. Simplified tectonic map of NNE Iran superimposed on GTOPO30 topographic image. The Khazar Fault (KHF) exists in the boundary zone between the South Caspian Basin (SCB) and Alborz Mountains. The Main Kopeh Dagh Fault (MKDF) forms a topographic boundary between Eurasia and Kopeh Dagh Mountains. The black rectangle in the inset box indicates the location of Figure 1 in the Middle East Alpine Collision belt. White arrows and their associated numbers represent the velocity vectors in mm yr -1 with respect to Eurasia

Figure 2 .

 2 Figure 2. A. Major faults in northeastern Iran superimposed on SRTM image showing leftlateral movement of NE-trending active faults in the Shahrud Fault System. The study area corresponds to the rectangle enlarged in Figure 3.

Figure 3 .

 3 Figure 3. A. The locations of different sites are identified in this Landsat (14 m resolution) image. Yellow circles are the location of sites where slip rates have been determined by dating of the offset alluvial fans. The abbreviations are as follows: AF: Abr Fault, KHF: Khij Fault. B. SPOT5 (5 m resolution) images focused on areas along the Abr and Khij faults (B-E). This Figure shows the enlargement of the termination zone of the Abr Fault. There are multiple small faults splayed in a rather wide area at the termination of the Abr Fault. C. Leftlateral deflection of streams is indicative of left-lateral movement of the Abr Fault. Fan boundaries are shown by dotted lines. D. The sharp escarpment of the Khij Fault which runs between volcanic (mainly Andesites) rocks and Quaternary. E. Three dimension view of the area on the basis of digital topographic maps with the scale of 1/20,000. The squares indicate

Figure 4 .

 4 Figure 4. A. SPOT5 image of a 12-km-long active reverse fault in the south of Nekarman. The dotted white rectangle is enlarged in Figure B. The location and direction view of Figure 6D is shown by the symbol. The trace of the fault is shown with small arrows. B. Topographic map (10 m contours) of this area is superimposed on the SPOT5 image. The A-B line represents the position of the topographic profile. C. The vertical displacement of the fault is estimated about 25 m.

Figure 5 .

 5 Figure 5. Structural map of the study area superimposed on the SRTM image, comprising the fault slip measurement sites. Trends of σ 1 (maximum stress axis) for the modern stress regime are presented. Sites are identified by numbers, see Table 2 for details. The inferred fault in the westernmost of the figure represents the Khazar Fault.

Figure 6 .

 6 Figure6. Lower hemisphere stereographic projections of the fault plane data together with inversion results determined by the inversion method proposed by[START_REF] Carey | Recherche des directions principales de contraintes associées au jeu d'une population de failles[END_REF]. The labels

Figure 7 .

 7 Figure 7. A. SPOT5 image focused on the Abr fan. B. Geomorphic map of Quaternary alluvial fans along the Abr Fault. Distinct surfaces of different ages are shown for the fan, from younger to older (Q1 to Q5). Solid line indicates the fault trace. Small squares show sampling locations on the alluvial fan surfaces. The associated ages are presented for three samples (The other samples did not have enough quartz for the process of in situ exposure 10 Be dating). There is an offset between the fan's feeder channel and the actual position of the fan axis. C. A backward slip of about 700 m restores the alignment of the fan's feeder channel and the fan axis. D: The rectangle area located in the west of the dating site shows left-lateral offset of a smaller stream channel for about 60 m. E. True left-lateral offset is considered about 42 m taking into account the active deflection of the stream.

Figure 8 .

 8 Figure 8. A. Topographic contour map (10 m contours) of Khij area displayed over SPOT5 image. A-B represents the position of the cross section. The small squares are indicative of collected samples. B. Topographic profile along the line A-B shows a vertical displacement of about 12 m along this segment of the Khij Fault. Sampling location and their associated in situ 10 Be ages are presented.

Figure 9 .

 9 Figure 9. A. The Google Earth image (resolution 3 m) clearly shows left-lateral displacement along the Khij Fault through stream channel offsets. B and C. left-lateral offset of ~ 400 m is achieved for this segment of the Khij Fault. Two fault planes nearly parallel with this segment of Khij Fault are shown in the lower stereographic projections.

Figure 10 .

 10 Figure 10. A. ENE looking view of the escarpment of the Khij Fault. B. NW looking view of the disconformity between the Quaternary and Neogene layers. The Quaternary surface is tilted and uplifted. This site is located in the vicinity of our dating site.

Figure 11 .

 11 Figure 11. In situ-produced 10 Be exposure ages and their associated uncertainties for the samples collected from the fan surface along the Khij Fault assuming no erosion and no

  black curve indicates the Gaussian age probability sum (Psum) and thin curves represent the age probability for each sample.

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 10 Figure 10

  . B. Fault map of the eastern Alborz and western Kopeh Dagh superimposed on 90 m SRTM (Shuttle-borne Radar Topography Mission) image of the area. Earthquake focal mechanisms are mostly based on the Harvard catalogue http://www.globalcmt.org/CMTsearch.html and McKenzie (1972) (see Table 1 for details).

Acknowledgments. This work was funded by the Dyeti and PNRN programs (INSU-CNRS).

We would like to thank M. R. Abbassi and M. Zare in International Institute of Earthquake Engineering and Seismology and A. Saidi, M. Ghorashi and M. Talebian in the Geological Survey of Iran for support and logistical assistance. SPOT images were provided thanks to the ISIS program (copyright CNES 2007 to 2008, distribution SPOT images S.A.). We are also grateful to R. Braucher for his help during sample chemical preparations and to the ASTER team for the cosmogenic nuclide concentration measurements (M. Arnold, G. Aumaître, K.

A c c e p t e d M a n u s c r i p t 26 temporal distribution of deformation in northern Iran: Geological Society of American Bulletin, v. 118, p. 1507-1521. Guest, B., Guest, A., Axen, G., 2007. Late Tertiary tectonic evolution of northern Iran: A case for simple crustal folding, Global and Planetary Change 58, 435-453. Hollingsworth, J., Jackson, J., Walker, R., Gheitanchi, M. R., Bolourchi, M. J., 2006. Strike slip faulting, rotation and along-strike elongation in the Kopeh Dagh Mountains, NE Iran. Geophys. J. Int. 166, 1161Int. 166, -1177Int. 166, . doi:10.1111Int. 166, /j.1365Int. 166, -246X.2006.02983..02983.x. Hollingsworth, J., Jackson, J., Walker R., Nazari, H., 2008. Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma, Geology, v. 36;no.10;p.763-766. Hollingsworth, J., Nazari, H., Ritz, J-F, Salamati, R., Talebian, M., Bahroudi, A., Walker, R., Rizza, M., Jackson, J., 2010 All angles are presented in degrees and date is as the usual convention (year/month/day). Figure 1B represents the focal mechanisms on the basis of these seismic data. The ID numbers refer to the label of each beachball in Figure 1B. It should be noted that the reported depths for individual earthquakes are not entirely reliable due to a poor azimuthal coverage of the worldwide station network. A c c e p t e d M a n u s c r i p t Principal stress axes (σ 1 > σ 2 > σ 3 ) and the stress ratio (R), defined as R = (σ 2 -σ 1 )/(σ 3 -σ 1 ) are presented in this table. N is the number of fault slip planes involved in the stress calculation. Site number 6 is resulted thorough the mentioned "fixed" solution (see text) and is marked by an asterisk. The references for the site numbers are Figures 5 and6. Note: 10 Be concentrations analytical uncertainties (reported as 1) include a conservative 0.5% external uncertainty based on long-term measurements of standards, a one sigma statistical error on counted 10 Be events, and the uncertainty associated with the chemical blanks correction. Two chemical blanks prepared with the samples yield similar 10 Be/ 9 Be ratio of 3.3±0.8×10 -15 and 3.4±0.7×10 -15 . For CRE age calculations, a modern sea-level highlatitude 10 Be production rate at of 4.5 ± 0.3 atoms/g/yr has been scaled for the sampling altitudes and latitudes using the scaling factors proposed by [START_REF] Stone | Air pressure and cosmogenic isotope production[END_REF]. An additional 6% production rate uncertainty is considered for CRE age calculation [START_REF] Stone | Air pressure and cosmogenic isotope production[END_REF].

Table 3 A c c e p t e d M a n u s c r i p