The growth of a mountain belt forced by base-level fall: Tectonics and surface processes during the evolution of the Alborz Mountains, N Iran Ballato, P., Landgraf, A., Schildgen, T. F., Stockli, D. F., Fox, M., Ghassemi, M. R., ... & Strecker, M. R. (2015). The growth of a mountain belt forced by base-level fall: Tectonics and surface processes during the evolution of the Alborz Mountains, N Iran. Earth and Planetary Science Letters, 425, 204-218. doi:10.1016/j.epsl.2015.05.051 10.1016/j.epsl.2015.05.051 Elsevier Accepted Manuscript http://cdss.library.oregonstate.edu/sa-termsofuse 1 The growth of a mountain belt forced by base-level fall: Tectonics and surface processes 2 during the evolution of the Alborz Mountains, N Iran 3 4 Paolo Ballato (1)*, Angela Landgraf (1), Taylor F. Schildgen (1), Daniel F. Stockli (2), 5 Matthew Fox (3,4), Mohammad R. Ghassemi (5), Eric Kirby (6), and Manfred R. Strecker (1). 6 7 (1) Institut für Erd- und Umweltwissenschaften, Universität Potsdam, 14476 Potsdam, 8 Germany 9 (2) Department of Geological Sciences, Jackson School of Geosciences, Austin, TX 10 78712, USA 11 (3) Department of Earth and Planetary, Science, University of California, Berkeley, 12 CA, USA. 13 (4) Berkeley Geochronology Center, Berkeley, CA, USA 14 (5) Research Institute for Earth Sciences, GSI, Tehran 13185-1494, Iran 15 (6) College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 16 Corvallis OR 97331-5503, USA 17 18 * Corresponding author: [email protected] 19 Accepted fro publication in EPSL 20 Abstract 21 The idea that climatically modulated erosion may impact orogenic processes has 22 challenged geoscientists for decades. Although modeling studies and physical calculations 23 have provided a solid theoretical basis supporting this interaction, to date, field-based work 24 has produced inconclusive results. The central-western Alborz Mountains in the northern 1 25 sectors of the Arabia-Eurasia collision zone constitute a promising area to explore these 26 potential feedbacks. This region is characterized by asymmetric precipitation superimposed 27 on an orogen with a history of spatiotemporal changes in exhumation rates, deformation 28 patterns, and prolonged, km-scale base-level changes. Our analysis suggests that despite the 29 existence of a strong climatic gradient at least since 17.5 Ma, the early orogenic evolution 30 (from ~ 36 to 9-6 Ma) was characterized by decoupled orographic precipitation and tectonics. 31 In particular, faster exhumation and sedimentation along the more arid southern orogenic 32 flank point to a north-directed accretionary flux and underthrusting of Central Iran. 33 Conversely, from ~ 6 to 3 Ma, erosion rates along the northern orogenic flank became higher 34 than those in the south, where they dropped to minimum values. This change occurred during 35 a ~3-Myr-long, km-scale base-level lowering event in the Caspian Sea. We speculate that 36 mass redistribution processes along the northern flank of the Alborz and presumably across 37 all mountain belts adjacent to the South Caspian Basin and more stable areas of the Eurasian 38 plate increased the sediment load in the basin and ultimately led to the underthusting of the 39 Caspian Basin beneath the Alborz Mountains. This underthrusting in turn triggered a new 40 phase of northward orogenic expansion, transformed the wetter northern flank into a new pro- 41 wedge, and led to the establishment of apparent steady-state conditions along the northern 42 orogenic flank (i.e., rock uplift equal to erosion rates). Conversely, the southern mountain 43 front became the retro-wedge and experienced limited tectonic activity. These observations 44 overall raise the possibility that mass-distribution processes during a pronounced erosion 45 phase driven by base-level changes may have contributed to the inferred regional plate- 46 tectonic reorganization of the northern Arabia-Eurasia collision during the last ~ 5 Ma. 47 48 Keywords: 49 Orogenic processes, surface processes, base-level fall, erosion, rock uplift, knickpoints 2 50 51 1. Introduction 52 Since the first suggestion that erosion can dictate the internal dynamics of orogenic 53 wedges (Davis et al., 1983), numerous analogue experiments (e.g., Koons, 1990; Hoth et al., 54 2006), numerical simulations (e.g., Willett, et al., 1993; Willett, 1999), and analytical 55 solutions to orogenic-wedge models (e.g., Whipple and Meade, 2006) have shown that 56 erosion of a tectonically active orogen may affect deformation processes in two ways: 1) 57 shifting deformation outward into the foreland or internally within the wedge to maintain a 58 critical orogenic taper (e.g., Davis et al., 1983; Willett et al., 1993); and 2) focusing rock 59 uplift in areas of high precipitation, where erosional processes are thought to be more efficient 60 (e.g., Willett, 1999; Thiede et al., 2004; Hodges et al., 2004; Reiners and Brandon, 2006). 61 While the first mechanism has proven difficult to demonstrate with field data (for a review 62 see Whipple, 2009), spatial correlations between long- to short-term erosion rates and modern 63 mean-annual precipitation in some orogenic systems have been presented as proof of a 64 coupling among erosion (which is thought to be controlled by climate, although such a 65 linkage is not yet fully understood; e.g., DiBiase and Whipple, 2011), topography, and 66 tectonics. Examples of such an interplay include the Washington Cascades (e.g., Reiners and 67 Brandon, 2006), the Southern Alps of New Zealand (e.g., Koons, 1990), the central and 68 southern sectors of the eastern central Andes (e.g., Bookhagen and Strecker, 2012), and the 69 southern Greater Himalaya (e.g., Hodges et al., 2004). Nonetheless, in nearly every orogen 70 where a coupling between climate and tectonics has been proposed, alternative data sets and 71 interpretations have argued against it, especially for large orogens with high heat flow, such 72 as the Himalaya (e.g., Burbank et al., 2003) and the Andes (e.g., Gasparini and Whipple, 73 2014). 3 74 Here, we explore the external controls on orogeny in the Alborz Mountains, the 75 deforming intracontinental orogen related to the Arabia-Eurasia collision zone (Figure 1). 76 According to GPS data and absolute gravity measurements, the northernmost deformation 77 front accommodates ~85% (~6 mm/yr) of the current total shortening across the range along 78 the Caspian Fault (also known as the Khazar Fault) coupled with surface uplift rates of 1 to 5 79 mm/yr (Figure 1; Djamour et al., 2010). Ongoing deformation therefore appears to be 80 concentrated along the northern orogenic margin, as shown by the distribution of shallow 81 crustal seismicity (e.g., Tatar et al., 2007; Donner et al., 2013). Furthermore, the orogen forms 82 an effective barrier to rainfall, with the northern flank receiving up to 2 m/yr sourced from the 83 Caspian Sea, and an arid to semiarid southern flank receiving < 0.5 m/yr (Figure 1). Elevated 84 topography of the range has existed at least since the Eocene, as documented by regional 85 stratigraphic relationships (e.g., Guest et al., 2006a; Rezaeian et al. 2012), while a stable C 86 and O pedogenic isotope record documents a climatic gradient across the range since at least 87 17.5 Ma (Ballato et al., 2010). In contrast, apatite fission track and zircon (U-Th)/He 88 thermochronometers (Figure 2; Guest et al., 2006b; Rezaeian et al. 2012; Ballato et al., 2013) 89 do not illustrate any long-term asymmetry in exhumation rates, as expected in active orogens 90 characterized by orographic precipitation (e.g., Willett, 1999). Taken together, these 91 observations suggest that if a present-day coupling among active deformation, topography, 92 and erosional processes (through orographic precipitation) indeed exists, it must have been 93 established relatively recently. 94 To reconcile these apparently contrasting observations and to explore what mechanisms 95 have dictated orogenic evolution, we combine analyses of river-channel morphology and new 96 apatite and zircon (U-Th)/He cooling ages with published information, including low- 97 temperature thermochronology data (Axen et al., 2001; Guest et al., 2006b; Rezaeian et al., 98 2012; Ballato et al., 2013), basin-fill histories from the southern Alborz foreland (Ballato et 99 al., 2008), the Caspian Sea (e.g., Allen et al., 2002; Brunet et al., 2003; Green et al., 2009), 4 100 and the intermontane Taleghan-Alamut basin (Guest et al., 2007), long-term paleoclimatic 101 records (Ballato et al., 2010), and decadal erosion rates (Rezaeian, 2008). Using these data, 102 we document temporal shifts in the locus of active deformation and exhumation across the 103 orogen, and compare them to both long-term and modern climate records to evaluate possible 104 feedbacks among tectonics, climate, and topography. Furthermore, we consider how the km- 105 scale base-level drop of the Caspian Sea between ~6 and 3.2 Ma (Forte and Cowgill, 2013; 106 Van Baak et al., 2012) may have influenced the orogenic evolution of the Alborz and 107 potentially also other mountain belts surrounding the Caspian. 108 109 2. Geologic setting 110 The Alborz Mountains of northern Iran are an intracontinental, double-verging orogen 111 within the Arabia-Eurasia collision zone, located between the relatively stable South Caspian 112 Basin and Central Iranian Block (Figure 1; e.g., Allen et al., 2004). Currently, the northern 113 margin of the central western Alborz range accommodates oblique plate convergence through 114 a combination of shortening (~ 6 mm/yr) and left-lateral wrenching (~2 mm/yr), while 115 deformation rates along the southern flank are < 1 mm/yr (shortening) to ~1 mm/yr (left- 116 lateral wrenching; Figure 1; Djamour et al., 2010). Although sparse, seismicity appears to 117 corroborate this pattern (Tatar et al., 2006; Aziz Zanjani et al., 2013; Donner et al., 2013; 118 Nemati et al., 2013).