8.1. Deformation Pattern of the Binalud Mountains
 Like other fold-and-thrust belts, e.g., Taiwan [Davis et al., 1983], India and Pakistan [Yeats, 1986], the Transverse Ranges [Namson and Davis, 1988; Bullard and Lettis, 1993] and the San Joaquin Valley of California [Keller et al., 2000], the topographic expression of the Binalud Mountains has been mostly produced by the ongoing thrust faulting and folding [e.g., Alavi, 1992]. In active fold-and-thrust belts, the deformation commonly migrates away from the highlands of the range toward the adjacent flanks. Interior faults of the system may become relatively inactive as the active tectonic processes are transferred to frontal fault systems [e.g.,Davis et al., 1983; Yeats, 1986, and references therein]. In the same way, but at a smaller scale, the topography and regional geomorphology of the Binalud Mountains are closely associated with geometry, kinematics and distribution of active faulting along the bounding faults (i.e., the Neyshabur Fault System and the Mashhad Fault Zone).
 In addition to these common modes of deformation known from the typical fold-and-thrust belts, the structural and geomorphic evolution of the Binalud Mountains are influenced by the right-lateral shear between central Iran and Eurasia. In its southeastern half, the Binalud appears to be an asymmetric mountain range characterized by a short and steeply sloping southwest flank versus a long and gently sloping northeast flank which is occupied by elongated and more linear drainage basins. This geometry can be explained by differing faulting mechanisms for the Neyshabur Fault System and the Mashhad Fault Zone (Figure 2a) producing differential vertical movements on both sides of the range. The southwest flank is uplifted due to reverse faulting along the Neyshabur Fault System, while the northeast flank is affected by strike-slip faulting along the Mashhad Fault Zone. Interestingly, these geomorphic and geometric characteristics are only observed within the southeastern half of the range where the Binalud Mountains are restrained between the overlapping range-bounding faults (Figure 16).
Figure 16. (top) A 3-D view of the Binalud Mountains based on SRTM topographic data representing general morphology of the mountains, major fault systems (thick white lines), as well as the vertical and horizontal slip rate estimates on both sides of the range. Slip rate for the Chakaneh Fault System (CFS) was taken fromShabanian et al. [2009b]. BIN, Binalud; MFZ, Mashhad Fault Zone; NFS, Neyshabur Fault System; MP, Mesozoic Plateau; EKD, eastern Kopeh Dagh; Eu, Eurasia. (bottom) Schematic cross section of the Binalud Mountains illustrating the inferred structure and the migration of the locus of active faulting within the Binalud restraining relay zone. Gray line marks inactive faults. The topographic profile is based on SRTM data.
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 The Binalud Mountains are deformed between the oblique-slip Neyshabur Fault System and the strike-slip Mashhad Fault Zone bounding the southwest and northeast sides of the range, respectively. Moreover, at the northwestern range border, the strike-slip Chakaneh Fault System (Figures 2a and 16) plays an important role transferring a part (≥2 mm/yr) of active strike-slip faulting between the Kopeh Dagh Mountains and the Neyshabur Fault System [Shabanian et al., 2009b]. On the northeast side of the range, the Mashhad Fault Zone slips right-laterally (1.3 ± 0.1 mm/yr) without direct structural connection to the Kopeh Dagh fault systems (Figure 2a). The right-handed arrangement of the Neyshabur Fault System and the Mashhad Fault Zone, which are characterized by the respective oblique-slip reverse and strike-slip fault kinematics, forms a soft-linked restraining step over. This can be regarded as a dextral relay zone in which the deforming southeastern half of the Binalud Mountains forms a crustal-scale pop-up structure (Figure 16). In this context, shortening (1.6–2.2 mm/yr) and uplift (∼2.8 mm/yr) is taken up by reverse component of faulting on the Neyshabur Fault System.
 Interestingly, the along-strike differences in the structural pattern have been recorded in the long-term geomorphic evolution of the range. The deeply incised topography of the southeastern half contrasts with the pristine geomorphology of the northwestern half in which the elevated remnant of a Mesozoic paleorelief exhibits its former erosional surface as a flat summit plateau (Figures 2a and 16). As it is characterized by remnants of upper Mesozoic rocks cropping out in several places (Figure 2a), we suggest that the Lower Triassic metamorphic core of the Binalud was initially covered by Upper Mesozoic marine carbonates. During Cenozoic, these rocks were totally removed from the southeastern half. The most preserved parts of the Mesozoic paleorelief is found far from the restraining relay zone, where the paleorelief is bounded by the strike-slip Chakaneh Fault System. The variation in the topography and geomorphology of the range implies a close relationship between denudation and long-term tectonic movements controlled by along-strike variations in the structural pattern of the range. The incised, high topography of the southeast Binalud Mountains resulted from shortening and the related uplifting within the restraining relay zone between the Neyshabur Fault System and the Mashhad Fault Zone (Figure 16). In contrast, there is no evidence of significant vertical movement in the northern half of the range affected by the pure strike-slip Chakaneh Fault System (Figures 2a and 16).
 Another possible explanation for this along-strike geomorphic difference is that the Mashhad Fault Zone is propagating to the northwest, and so has not yet disrupted the erosion surface (Mesozoic paleorelief) farther northwest. This seems consistent with the fact that deformation is localized on a single strand to the southeast on a more mature fault zone, and is distributed on various less mature strands to the northwest. However, we lack adequate data to choose between these two possibilities, which are not mutually exclusive and may, in fact work together. Further data will better constrain the situation and favor one (or more) possibilities over others.
 At the scale of the Binalud restraining zone, the basinward migration of the depositional zones of Q3, Q2, and Q1 alluvial fans indicate the migration of deformation locus away from the highlands of the range such that the youngest structures are at the edge of the range. During the deposition of Q3 alluvial fans, the front of the Binalud Mountains corresponded to the Barfriz Fault. As the mountain front developed, new faults formed basinward and a new mountain front has developed. Older alluvial fans were tilted, faulted, and incorporated within the foothills domain (Figures 3, 4b, and 5c). The older, now intermountain front, i.e., the Barfriz Fault Zone, has been abandoned in the interior of the range, and new fans were developed basinward. Late Holocene fault activities (≤5 ka) are expressed in vertical fault offsets recorded by Q1 alluvial fans which were formed downhill from the southwestern extension (external limit) of the Buzhan Fault Zone (e.g., the Somaeh Fault). The youngest fault activity is represented by basinward development of fault ruptures (Figure 9) or up-warped alluvial fans indicating the active front of deformation ∼4.8 km southwest of the older mountain front (Barfriz Fault Zone). These observations clearly reveal the basinward migration of deformation locus and the across-range growth of the Binalud Mountains during the last ∼100 ka.
8.2. Implications for the Geodynamics of Northeast Iran
 As for the Neyshabur Fault System, we estimated a horizontal slip rate of 2.4 ± 0.5 mm/yr which is an average of slip rates determined from the cumulative offsets measured on the Qadamgah Fault and the Barfriz Fault Zone. The total 290 ± 50 m vertical deformation deduced from the geomorphic reconstruction of Q3 fan surfaces (Figures 8a and 8b) yields an uplift rate of 2.8 ± 0.6 mm/yr averaged over 105 ± 14 ka. If the fault geometry of N135°E/60 ± 5°NE (Figures 7c, 9e, and 12b) remains constant with depth, this uplift is accounted by a 1.6 ± 0.4 mm/yr shortening rate perpendicular to the Binalud Mountains. But, if the thrusts in the Buzhan Fault Zone flatten with depth then the horizontal convergence rate would be higher than this value (see Hollingsworth et al.  for more information). In this case, the shortening rate across the Neyshabur Fault System will be the sum of vertical slip rates (1.4 ± 0.4 mm/yr) of the Buzhan Fault Zone and the shortening rate (0.8 ± 0.3 mm/yr) across the Barfriz Fault, i.e., 2.2 ± 0.7 mm/yr. As indicated by Hollingsworth et al. , the geometry of geological structures across the fault system allows the differentiation between these two models, and to determine the correct shortening rate estimate. Actually, given the asymmetric SW verging fold geometry between the Barfriz and Buzhan Fault Zones (Figure 5c), it is not very likely that the Buzhan Fault Zone flattens at shallower depth above a horizontal décollement. However, the fault dips may change with depth (do not confuse with flattened faults). In such a case, we incorporate both shortening rate estimates as upper and lower bounds, yielding 1.6–2.2 mm/yr. These rate estimates are, of course, higher than the shortening rate (0.3–1.0 mm/yr) proposed by Hollingsworth et al. , who only considered the Eyshabad Fault.
 The range-parallel right-lateral (2.4 ± 0.5 mm/yr) and range-perpendicular shortening rates (1.6–2.2 mm/yr) allowed us to estimate the horizontal motion of central Iran relative to the Binalud Mountains at a late Quaternary rate of 2.9 ± 1.0 mm/yr in the N350 ± 13°E direction. In the same way, the addition of the 1.3 ± 0.1 mm/y lateral slip rate on the strike-slip Mashhad Fault Zone yields the overall motion of central Iran relative to eastern Kopeh Dagh at a rate of 4.0 ± 1.3 mm/yr, in N340 ± 8°E direction.
 The regional integration of the slip rates gives a coherent image of the deformation field in northeast Iran (Figure 17). As suggested by Shabanian et al. [2009a], almost 80% (∼6 mm/yr) of the total ∼8 mm/yr of the central Iran–Eurasia northward motion is localized along the Bakharden-Quchan Fault System in Kopeh Dagh (Figure 17). Our slip rate estimates indicate that the Binalud Mountains take up ∼3.8 mm/yr (∼60%) of this northward motion through dextral oblique-slip to strike-slip faulting localized on the SW and NE sides, respectively. It is likely that the remaining part (∼40%) of the overall deformation is distributed on other dextral and reverse faults in eastern Kopeh Dagh such as the Hazarmasjed and Kashafrud Faults (Figure 17).
Figure 17. Regional integration of geological fault slip rates in northeast Iran. Slip rates and associated uncertainties (within parentheses) are followed by the time over which the rates were estimated. White boxes from Shabanian et al. [2009a] (36Cl ages); light gray boxes from Shabanian et al. [2009b](Ar-Ar ages); dark gray boxes from this study (10Be ages); the black box from Javidfakhr et al. [2011b] (10Be age). White arrows are the geologically derived northward motion (in mm/yr) of central Iran relative to eastern Kopeh Dagh. Abbreviations: MKDF, Main Kopeh Dagh Fault; BQFS, Bakharden-Quchan Fault System; DF, Doruneh Fault; MTZ, Meshkan Transfer Zone; KF, Kashafrud Fault; HF, Hazarmasjed Fault; EKD, eastern Kopeh Dagh; WKD, western Kopeh Dagh.
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8.3. Integrating Data-Derived Results Within Preexisting Tectonic Models
 The range parallel oblique-slip to strike-slip faulting on both sides of the Binalud Mountains shows that the northward motion of central Iran relative to Eurasia has not wholly been absorbed by crustal shortening and thrust faulting in the range [e.g.,Berberian and Yeats, 1999; Hollingsworth et al., 2010]. This knowledge adds another piece to the geodynamic puzzle of northeast Iran (1) indicating the significant contribution of the Binalud faults to the accommodation of active deformation in northeast Iran and (2) suggesting that the Mashhad and Neyshabur Faults (bounding the Binalud Mountains) define the continuation of the northeastern Arabia-Eurasia intraplate boundary beyond Kopeh Dagh [e.g.,Shabanian et al., 2009a]. This also strengthens the suggestion that the Miocene–early Pliocene shortening and crustal thickening, at the scale of northeast Iran, were dominated by the present-day oblique-slip to strike-slip faulting localized along well-defined fault systems [Shabanian et al., 2009b].
 The kinematics of active deformation in northeast Iran has been a matter of debate [see Shabanian et al., 2009b, and references therein; Siame et al., 2009]. The last kinematic model has been proposed by Hollingsworth et al.  describing the geometry and kinematics of northeast Iran by oroclinal folding. The Binalud Mountains forms the eastern limb of the oroclinal fold, with small contribution (minor thrust faulting) to accommodate active deformation [Hollingsworth et al., 2010]. This model is based on the simple assumption that the Alborz Mountains, with a complex Cenozoic tectonic history [e.g., Stöcklin, 1974; Berberian and King, 1981; Alavi, 1992; Axen et al., 2001; Ritz et al., 2006b; Zanchi et al., 2006, Guest et al., 2007; Shabanian et al., 2009b; Ballato et al., 2011; Javidfakhr et al., 2011a; Sheikholeslami and Kouhpeyma, 2012], were initially a uniform E-W mountain range. Interestingly, this model requires that the geometry and kinematics of the east and west oroclinal limbs (i.e., deformation zone boundaries) evolve with time as the oroclinal folding develops. Such significant variations in the geometry of the Alborz and Kopeh Dagh deformation zones directly discard the main assumption (constant zone boundaries) of the previous model [Hollingsworth et al., 2006] on which the present model is built. Consequently, this annuls both model-derived slip rates and ages estimated by the authors. Additionally, a simple but fundamental problem with this model is that folding at the scale of the Alborz Mountains (oroclinal) due to the northward motion of central Iran–Eurasia requires an “indenter”, as is the case of the Talesh–western Alborz mountains indented by the South Caspian rigid block [seeBerberian, 1983; Djamour et al., 2010; Shabanian et al., 2012]. In other words, the South Caspian Basin persists against the northward motion of central Iran, deforming the western portion of the Alborz range from an “assumed” E-W orientation. But, it is very unlikely that the deforming continental domain between the ∼E-W Doruneh Fault, as the southern boundary for the northeast Iranian deformation domains, and the Allah Dagh–Binalud Mountains behaves as an indenter, folding the mountains. A nearly similar tectonic model was proposed byWalker and Jackson  to explain the ∼70 km northward curvature of the Doruneh Fault due to the northward motion of central Iran and Lut block relative to Eurasia. The structural and geomorphic studies carried out by Farbod et al.  reveal that neither kinematics nor geometry of the Doruneh Fault can be described by such a structural model.
 Given the diachronous formation to closure of the eastern and western Kopeh Dagh basins [Afshar Harb, 1979; Lyberis and Manby, 1999] and the lack of thrust faulting along the Mashhad Fault Zone since the Devonian time [Majidi, 1978], we suggest that the overall shape of the Kopeh Dagh and Binalud mountains has principally been controlled by the former geometry of the north (Eurasia) and south (central Iran) margins. That is, the mountain belt never had an E-W linear orientation. The homogeneity, at the scale of northeast Iran, of the Miocene-Pliocene and Quaternary stress states [Shabanian et al., 2010, 2012; Javidfakhr et al., 2011a] argues for the fact that no significant change has occurred in the orientation of the belt since at least the Miocene. Otherwise, the stress state should significantly change in either limb, and in the hinge area of the “inferred” oroclinal. Afterward (3–5 Ma), the geometry of the belt has been slightly changed and adjusted accommodating strike-slip faulting oblique to the Alborz and Kopeh Dagh mountain ranges [Shabanian et al., 2009a, 2009b; this study]. In the active tectonic context, reverse faulting is localized along the WNW trending faults [e.g., Shabanian et al., 2010] that take up the reminders of deformation not transferred by strike-slip faulting. The fact that the Binalud mountains is deformed as a soft-linked restraining relay zone (section 8.1) favors the northward translation of central Iran with respect to Eurasia.