This interpretation contradicts the previous hypothesis of constant slip along the entire length of the NTF. Such a slip distribution pattern may explain the existence of smaller (~300 m) Pliocene-Quaternary cumulative dextral displacements along the SE segment of the fault than the measured cumulative displacements along the NW segment (~800 m) of the NTF. 2003) conducted the first paleoseismological study on the northwestern segment of the NTF to characterize both its kinematics and seismic behavior.
In this context, a detailed study along the SE segment of the NTF is essential to better define the regional seismic hazard. The results of our systematic morphotectonic, seismotectonic and paleoseismological field studies (see Solaymani Azad, 2009) complemented those of previous seismotectonic and paleoseismological studies along the NW segment of the NTF (Karakhanian et al Hessami et al., 2003a). Berberian (1997) and Karakhanian et al. 2004) suggested that the NW-SE striking NTF can be considered the eastern extension of the GSCF.
This estimate is higher than the ~2 mm yr-1 geologic slip rate found in the Holocene along the NW segment of the NTF (Karakhanian et al., 2004). The NW and SE terminations of the NTF are marked by the Mishu and Bozghush sets, respectively (Figures 1 and 2).
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Geomorphology and kinematics of the NTF
- The Northwestern sector of the TFS
- The central sector of the TFS
- The Southeastern sector of the TFS
- Pattern of active deformation along the TFS
Geomorphic evidence of dextral strike-slip along the North Mishu fault is noticeable southwest of the fold. The minimum horizontal geological slip rate along the NW segment of the NTF is about 2 mm l-1 (Karakhanian et al., 2004). The smallest dextral geomorphic offset observed along the NW segment of the NTF is about 4–5 m (Hessami et al., 2003a).
At least three right branches can be observed along the SE segment of the NTF. These branches of the main strike-slip fault segment create two steps to the right, one to the west of the. The geomorphology of the fault scarp is consistent with a NE-dipping reverse to the thrust fault.
The SE extension of the NTF terminates southeast of Bostanabad, where the fault divides into two branches. The main strand gradually expands to the east and follows the southern flank of the Bozghush range.
Paleoseismological study along the SE segment of the NTF
- Site 1
- T1bis and T1 Trenches
- T2 trench
- Site 2
- Dating analysis
The maximum dextral shift of 300 m is restored using satellite imagery and aerial photographs by matching the streams and ridges on both sides of the fault track (Figures 6 and 8). However, almost pure right-lateral strike-slip fractures and more recent deformations are located along the main fault track of the NTF (Section 4.2; see also Figures 6 and 8). This site is located 1.2 km east of Lake Ghurugol, where the NTF zone intersects the well-developed Quaternary alluvial fans overlying the southern flank of an area of Eocene volcanoes, parallel to the fault track (Fig. 6 and 8). .
In addition, the detailed DEMs together with our field surveys allowed us to better determine the surface geometry of the fault branches and the associated fold geometry (Fig. 11). Along the main trace of the fault, we found several offset markers indicating recent horizontal displacements with typical offsets of ~7 m, which may correspond to the occurrence of the most recent seismic event (Figure 9). Because elements of unit 2a appear to be interbedded with unit 2b, unit 2b is interpreted as a lateral sedimentary variation of salt pond deposits (Fig. 15).
Recently, unit 2 has been successively cut off by fracture events in the main fracture zone between meters 4–5.5 and 5.5–7 in trenches T1bis and T1, respectively (Figures 12 and 13). Sedimentary units along the entire length of the trench wall remain horizontal except near the main fracture zone, indicating that the dominant slip component is strike-slip. Within trenches T1bis and T1, the Miocene-Pliocene rock is located south of the main fault zone, while the rock at site 2 is located on the north side of the fault track.
In such a case, the secondary vertical component of the fracture and the orientation of the uplifted block could vary along the fracture trace. To identify the Paleoquakes that ruptured the east-central sector of the TFS, we focused our analysis on site 1 and primarily within the T1bis and T1 trenches. To map the chronology of the paleoearthquakes, we reconstructed the geometric and sedimentary evolution of the fracture zone (Figures 15 and 18A-F).
18A shows the situation at the time of the surface fault in connection with the second strong earthquake. 18D shows the restoration of the Earth's surface to its position just after the penultimate strong earthquake. We also found a seismic event in the T2 trench with an age older than 18.5±2 kyr (unit 5 is the post-event horizon), which could be associated with one of the oldest seismic events recognized in the T1bis- and the T1 trenches.
Discussion and conclusion
This may explain the differences in coseismic slip rates and earthquake characteristics of the two fault segments. This decrease in the eastward slip rate may imply a more distributed fault zone along the SE segment. It appears that much of the deformation along the SE segment of the fault is accommodated by dextral scaling, which results in lateral propagation of the fault.
In fact, the vertical component of the slip vector along the SE sector of the NTF is slightly more than along the NW segment of the fault. This diffuse deformation may also imply smaller cumulative Pliocene-Quaternary dextral displacements and slower seismic deformation along the SE segment of the fault than along the NW segment. Moreover, the earthquakes that rupture the SE segment have stronger magnitudes (M~7.7), i.e. longer return periods, than those that occur in the NW segment of the NTF (M~7.5).
All these considerations emphasize the differences in seismic behavior and especially in coseismic slip rate variations along the NW and SE segments of the NTF. Southwest of the fold, a major dextral strike-slip fault (the North Mishu fault), located in the extension of the NTF, clearly offsets the drainage network. Near the Khajeh-Marjan village (see Fig. 4 for location), the NW segment of the NTF crosses several generations of alluvial fans (the chronological classification is based on shear erosion).
In addition, the drainage pattern on both sides of the lake indicates a paleoflow direction to the northeast. Eastward view of the main fault plane west of Lake Ghurugol (between Shebli and the lake in Figure 4). Morphostructural map and simplified N–S cross-section of the eastern end of the TFS within the Bozghush area (see Fig. 2 for location).
Much of the NTF's strike-slip component is accommodated along the various oblique inverse faults, primarily E–. Evidence of recent right-lateral shifts along the SE segment of the NTF near sites 1 and 2. The pierce points are marked by black arrows on either side of the fracture trace.
The fault lines bounding both sides of the hill have eroded the Late Quaternary (LQ) deposits. Detailed topographical map based on a digital elevation model (DEM) derived from differential global positioning system (DGPS) survey of the trench site 1. Reconstruction of key sedimentary unit geometries after the oldest paleo event found in T1bis and T1 trenches.
The width of the fault zone is about 5 m near the surface and less than 2 m at a depth of about 5 m.
