Middle to late Cenozoic basin evolution in the western Alborz Mountains: Implications for the onset of collisional deformation in northern Iran Bernard Guest Department of Earth and Space Sciences, University of California, Los Angeles, USA. Now at Geology Section, Department of Geological and Environmental Sciences, Ludwig-Maximilians-University, Munich, Germany. Brian K. Horton Department of Earth and Space Sciences, University of California, Los Angeles, USA. Now at Institute for Geophysics and Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, Texas, USA. Gary J. Axen Department of Earth and Space Sciences, University of California, Los Angeles, USA. Now at Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA. Jamshid Hassanzadeh Department of Geology, University of Tehran, Tehran, Iran. William C. McIntosh New Mexico Geochronology Research Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA. Guest, B., B. K. Horton, G. J. Axen, J. Hassanzadeh, and W. C. McIntosh (2007), Middle to late Cenozoic basin evolution in the western Alborz Mountains: Implications for the onset of collisional deformation in northern Iran, Tectonics, 26, TC6011, doi:10.1029/2006TC002901. CONTENTS: 1. Supplement to Text Geological background and regional stratigraphy References Figure caption Figure S1 2. Geochronology Introduction Analytical methods Summary of Results References Data tables (Tables S1 and S2) Data plot (Figures S2 – S18) 1. SUPPLEMENT TO TEXT Geologic Background and Regional Stratigraphy Northern Iran recorded a complex history of passive margin sedimentation, active margin magmatism, and extensional and contractional tectonics from Neoproterozoic to present [Stocklin, 1968, 1974; Stocklin and Setudehnia, 1977; Berberian and King, 1981; Berberian, 1983; Davoudzadeh et al., 1997). To understand the structural and stratigraphic evolution of the middle to late Cenozoic basin system described in this paper requires an explanation of the regional stratigraphy of the Alborz Mountains. The geologic column in the Alborz consists of Neoproterozoic through Eocene-Oligocene sedimentary, metasedimentary, and limited igneous rocks. These rocks are involved in structures associated with the Oligocene-Miocene basin fill of the Taleghan and Alamut synclines (Figure 2) and served as the dominant sediment sources to this middle to late Cenozoic basin system. The formations described below, except where noted, contain diagnostic lithologies recognizable as clasts in Oligocene-Miocene conglomerates (Figure S1). The oldest rocks exposed in the Alborz Mountains belong to the Neoproterozoic Kahar Formation (Figure S1), which consists mainly of argillaceous to sandy micaceous slate and shale [Stocklin and Setudehnia, 1977). The base of the Kahar Formation and its presumed underlying crystalline basement are not exposed in the Alborz [Annells et al., 1977; Stocklin and Setudehnia, 1977; Vahdati Daneshmand, 1991]. A conformable sequence of upper Neoproterozoic through Ordovician marine carbonate and clastic rocks overlies the Kahar Formation, and is divided into five formations (Figure S1). The most identifiable rocks from this section are yellow, white, and grey dolostone of the Soltanieh and Barut formations, red to pink sandstone and siltstone of the Lalun and Zaigun formations, white quartz arenite from the "top quartzite" cap of the Lalun Formation [Stocklin, 1968], and red to tan dolostone and limestone of the Mila Formation. The lower Paleozoic sequence is disconformably overlain by the Upper Devonian through Lower Carboniferous Geirud and Mobarak formations [Stocklin and Setudehnia, 1977]. The Moberak Formation is composed of distinctive, highly fossiliferous blue-grey limestone, and is abundantly exposed directly north of the Alamut basin, but pinches out southward (Figure S1) [Annells et al., 1975a, 1975b; Vahdati Daneshmand, 1991]. The Upper Triassic to Middle Jurassic Shemshak Formation unconformably overlies units ranging from Cambrian to Triassic in age [Stocklin and Setudehnia, 1977]. In the Alborz this formation is composed of a northward-thickening succession (Figure S1) of lithic-rich pebbly sandstone that fines upward and laterally into a thick section of alternating calcareous sandstone, siltstone, and shale. Clasts originating from the Shemshak Formation and the conformably overlying Jurassic Dalichai and Lar formations are not readily identifiable in Oligocene-Miocene basin fill. Orbitolina-bearing grey limestone of the Cretaceous Tiz Kuh Formation overlies Jurassic units and is easily identified as clasts in younger conglomeratic rocks. Unconformably overlying the Tiz Kuh Formation is a laterally variable, mid- to Upper Cretaceous sequence of limestone, marl, volcanic, and volcaniclastic rocks that thickens northward from a depositional pinchout in the southern Alborz to several hundred meters of section along the N flank of the range (Figure S1). In the S central Alborz, the Paleocene-Eocene Fajan conglomerate overlies folded Cretaceous and older rocks in angular unconformity. This red polymict conglomerate is similar in appearance to Oligocene-Miocene fill of the Taleghan and Alamut basins (Figure 2) but is distinguished by its limited exposure area and stratigraphic position between fossiliferous carbonates of the underlying Cretaceous section and overlying Eocene Ziarat Formation. It is possible that Fajan conglomerate clasts were recycled into younger basin fill conglomerates but no polymict clasts have been identified in younger strata. The Ziarat Formation consists of distinctive, beige to yellowish nummulitic (foraminiferal) limestone beneath the voluminous Eocene Karaj Formation of the western Alborz Mountains [Stocklin and Setudehnia, 1977]. This formation crops out south of the Taleghan and Alamut basins, but is not present to the north (Figure S1). The Eocene Karaj Formation crops out north and south of the Taleghan and Alamut basins and is the primary contributor of clasts to the Oligocene-Miocene basin fill in the region. The Karaj Formation is laterally variable, consisting of a thick (>3 km) succession of interstratified tuffaceous shale, green tuff, nummulitic limestone, and volcaniclastic turbidite deposits overlain locally by a thick (>500 m) sequence of andesitic lavas (Figure S1). Most Karaj Formation lithologies are easily recognized as clasts. The original regional extent of the Karaj Formation is unclear. Most authors argue, based on present outcrop distribution and the lack of Karaj Formation rocks in the south Caspian basin, that the Karaj Formation had a depositional pinchout near the N edge of the Alborz [Stocklin, 1974; Stocklin and Setudehnia, 1977; Berberian, 1983; Hassanzadeh et al., 2004]. Alternatively, the Karaj Formation may have existed along the N flank of the Alborz but was eroded away during a recent episode of deformation and uplift. Oligocene-Miocene strata exposed in the Taleghan and Alamut basins (Figure 2) are divided here into the informally named Gand Ob unit and unconformably overlying Narijan unit (Figure S1), described in detail below. In general, the uppermost lavas of the Eocene Karaj Formation are disconformably overlain by shallow marine, lacustrine, and fluvial strata of the Gand Ob unit in the east and by younger alluvial fan, fluvial, and lacustrine strata of the Narijan unit in the west [Guest, 2004]. These units of Oligocene- Miocene age were folded then overlain in angular unconformity by undeformed Pliocene and younger gravels preserved in high terraces up to 300 m above the modern Taleghan and Alamut rivers. The terrace deposits consist of poorly sorted fluvial gravels, sands, and silts. Along the southern range front, Quaternary alluvial fan gravels are cut and locally overturned by active thrust faults. References Annells, R. N., Arthurton, R. S., Bazley, R. A., and Davies, R. G., 1975a, Explanatory text of the Qazvin and Rasht Quadrangles Map: Tehran, Geological Survey of Iran. Annells, R. N., Arthurton, R. S., Bazley, R. A., and Davies, R. G., 1975b, Geological Quadrangle Map of Iran, Qazvin and Rasht Sheet: Geological Survey of Iran, scale 1:250,000. Annells, R. S., Arthurton, R. S., Bazley, R. A. B., Davies, R. G., Hamedi, M. A. R., and Rahimzadeh, F., 1977, Geological map of Iran, Shakran sheet 6162: Geological Survey of Iran, scale 1:100,000. Berberian, M., 1983, The southern Caspian: A compressional depression floored by a trapped, modified oceanic crust: Canadian Journal of Earth Science, v. 20, p. 163 183. Berberian, M., and King, G. C. P., 1981, Towards a paleogeography and tectonic evolution of Iran: Canadian Journal of Earth Sciences, v. 18, p. 210-265. Davoudzadeh, M., Lammerer, B., Weber-Diefenbach, K., 1997. Paleogeography, stratigraphy, and tectonics of the Tertiary of Iran. Neues Jahrbuch fuer Geologie und Pala¬ontologie, Abhandlungen, v. 205, p. 33-67. Guest, B., 2004, The thermal, sedimentological and structural evolution of the central Alborz Mountains of northern Iran: Implications for the Arabia-Eurasia continent-continent collision and collisional processes in general [Ph.D. Dissertation]: University of California, Los Angeles, 292 p. Hassanzadeh, J., Axen, G., Guest, B., Stockli, D.F., and Ghazi, A.M., 2004, The Alborz and NW Urumieh-Dokhtar magmatic belts, Iran: Rifted parts of a single ancestral arc: Geological Society of America Abstracts with Programs, v. 36 (5), p. 434. Stocklin, J., 1968, Structural history and tectonics of Iran: A review: American Association of Petroleum Geologists Bulletin, v. 52, p. 1229-1258. Stocklin, J., 1974, Northern Iran: Alborz Mountains, in Spencer, A. M., ed., Mesozoic-Cenozoic orogenic belts; data for Orogenic Studies; Alpine-Himalayan orogens: Special Publication: London, Geological Society of London, p. 213-234. Stocklin, J., and Setudehnia, A., 1977, Stratigraphic lexicon of Iran: Geological Survey of Iran, Report 18, 376 p. Vahdati Daneshmand, F., 1991, Amol; Geological quadrangle map of Iran: Geological Survey of Iran, scale 1:250,000. 2. GEOCHRONOLOGY Introduction Fifteen volcanic/hypabyssal rocks from the Alborz Mountains, Iran were submitted for 40Ar/39Ar dating by Drs. Bernard Guest and Gary Axen. It was hoped that the 40Ar/39Ar ages will help constrain the tectonic evolution of the Alborz Mountains. The table below lists the material/mineral prepared from each sample: Groundmass concentrate Biotite Hornblende 19-133-1 19-141-2A 19-57-2 19-135-1 19-141-2B 19-141-1 19-137-1 20-49-3 3-87 19-141-2B BG-4 84-2A 3-88 20-49-2 BG-4 84-2B BG-4 86-3 3-89 40Ar/39Ar Analytical Methods and Results The groundmass concentrate, biotite and hornblende samples were analyzed by the furnace incremental heating age spectrum 40Ar/39Ar method. Abbreviated analytical methods for the furnace sample is given in Table S1. Details of the overall operation of the New Mexico Geochronology Research Laboratory are provided in the Appendix. Figures S2-S17 show the age spectrum and inverse isochron yielded by the groundmass concentrates, biotites and hornblendes. A summary of the preferred ages yielded in this study is shown in Table S1. Each of the six groundmass concentrate samples yielded a slightly to somewhat discordant age spectrum. For 19-133-1 (Figure S2), the first two heating steps (steps A and B) are significantly older (age ~60 Ma) than the remainder of the heating steps (age ~32 Ma), which exhibit a concordant age distribution. The radiogenic yields increase gradually from a low of 2.3% for the lowest temperature heating step to greater than 84% for the 1250¡C (the 1650 deg C or fusion step typically has a lower radiogenic yield resulting from a higher extraction line blank). The K/Ca values range from 0.03 (step I) to 0.8 (step G), which are consistent with a basaltic groundmass concentrate. A weighted mean value for the flattest portion of the age spectrum (steps C through H; 85.7% of the cumulative 39ArK released) yields an apparent age of 32.76±0.44 Ma (two sigma) with an unacceptable MSWD of 3.3. The inverse isochron for 19-133-1 does however yield an acceptable MSWD of 1.8. The age yielded by the inverse isochron is 32.45±0.32 Ma with a 40Ar/36Ar intercept of 300±2. Groundmass concentrate 19-135-1 (Figure S3) yields an age spectrum that is also discordant for the first few heating steps. While steps A and B are anomalously older than the remainder of the age spectrum, steps C and D are anomalously younger than the remainder of the age spectrum. This undulatory behavior is confined to the heating steps below 800 deg C, as those steps above 800 deg C are 90% of 39ArK released. Radiogenic yields range from about 40% to greater than 90% for the majority of the spectrum. K/Ca values are initially high (>10), but then drop below 10 for those temperature steps 1180 deg C and higher. The weighted mean age for the flattest portion of the age spectrum (steps C through K) is 6.87±0.08 Ma (95.6% of the 39ArK released) with a MSWD of 1.5. The inverse isochron yields a very similar age of 6.89±0.03 Ma (40Ar/36Ar intercept=294±2; MSWD=1.3). Biotite 19-141-2B yields an age spectrum (Figure S9) very similar to that of 19-141-2A. However, while the radiogenic yields are only slightly higher for 19-141-2B, the K/Ca values are approximately 4 times higher than those observed for 19-141-2A. The weighted mean age for steps D through K is 6.89±0.05 with 90.9% of the 39ArK released (MSWD=1.4). The inverse isochron yields an age of 6.91±0.06 Ma (40Ar/36Ar intercept=294±2; MSWD=1.5). The age spectrum for biotite 20-49-3 (Figure S10) does exhibit some discordance, but still yields a precise weighted mean age. The age spectrum discordance is mainly confined to the lower temperature heating steps (<1180 deg C) where ages range from 0.23 to 0.92 Ma (excluding the A step). Following the anomalous behavior at low temperatures, the age spectrum becomes more isochronous where ages range from 0.41 to 0.59 Ma. The radiogenic yields for 20-49-3 are low and consistent for a significant quantity of the gas released, ranging from 1.3 to about 10%. Only steps J and K have radiogenic yields greater than 20%. The K/Ca values are greater for the lower temperature steps (~12) than the high temperature steps (<6). A weighted mean for the flattest portion of the age spectrum (steps G through K) yields an age of 0.51±0.03 Ma with 86.8% of the 39ArK released (MSWD=2.7). The inverse isochron yields an age of 0.49±0.08 Ma (40Ar/36Ar intercept=296±6; MSWD=2.2). The final two biotite samples from this study (BG-4 84-2A and BG-4 84-2B; Figures S11 and S12, respectively) yield results very similar to one another. In both cases, initial ages are anomalously younger than subsequent ages. For BG-4 84-2A, the youngest age (excluding the A step) is 4.69 Ma, while BG-4 84-2B's youngest age is 4.82±0.86 Ma. Radiogenic yields and K/Ca values for the two samples are also very similar, positively correlating to the shape of the age spectra. The weighted mean age for BG-4 84-2A is 7.06±0.08 Ma (steps E through K) with 89.0% of the 39ArK released (MSWD=2.9). The weighted mean age for BG-4 84-2B is 7.31±0.07 Ma (steps D through K) with 92.0% of the 39ArK released (MSWD=1.5). The inverse isochron for BG-4 84-2A yields an age of 7.07±0.13 Ma (40Ar/36Ar intercept=295±10; MSWD=2.6). The inverse isochron for BG-4 84-2B yields an age of 7.33±.010 Ma (40Ar/36Ar intercept=293±6; MSWD=2). The hornblende samples from this study yield poor 40Ar/39Ar results when compared to the groundmass concentrates and the biotites. In nearly every case, the hornblendes degassed the bulk of their argon in only 2 to 3 heating steps. For hornblende 19-57-2 (Figure S13), approximately 85% of the 39ArK was released in steps H and I. The initial heating steps (A through G) yielded ages ranging from 2.50 to 21.36 Ma. Radiogenic yields range from 9.6 to 33.5%. K/Ca ratios range from 0.006 to 2.4. The weighted mean age for steps H and I is 1.39±0.21 Ma with a MSWD of 9.9. The inverse isochron yields an age of 0.69±2.3 Ma with a 40Ar/36Ar intercept of 360±246 (MSWD=42). Hornblende 19-141-1 (Figure S14) also degassed predominantly in two heating steps, but its age spectrum does not exhibit the discordance present in 19-57-2. Steps H and I contain almost 95% of the 39ArK released for a weighted mean age of 6.71±0.08 Ma (MSWD=0.2). The radiogenic yields for those steps containing greater than 2% of the 39ArK are greater than 33%. The K/Ca ratios for all of the steps is consistent at approximately 0.15. The inverse isochron yields an age of 6.74±0.08 Ma (40Ar/36Ar intercept=289±3; MSWD=1.2). Like the 19-141-2B groundmass concentrate, the 3-87 hornblende age spectrum (Figure S15) is distinctly saddle-shaped. Although the initial ages (steps A through C) appear to be consistent at approximately 9.5 Ma, the following 3 steps (D through F) immediately spike to ages in excess of 11 Ma. Steps G through I return to a comparatively isochronous distribution with a weighted mean age of 2.85±0.83 (77.4% of the 39ArK released) and a high MSWD of 41.9. The final two heating step once again increase in age to greater than 30 Ma. Radiogenic yields and K/Ca values are positively correlated to the shape of the age spectrum. The results for the inverse isochron are very discordant when all of the heating steps are plotted (MSDW=940). With only the three plateau steps plotted, the isochron yields an age of 3.5±1.8 (40Ar/36Ar intercept=274±43; MSWD=9.4). The age spectrum results for 3-88 (Figure S16) exhibit some anomalous behavior at the intermediate heating steps. Steps D through G are slightly older and less precise than the rest of the age spectrum. However, this gas only represents ~10% of the total 39ArK released and therefore does not significantly influence the results of a weighted mean from steps B through I (age=0.34±0.32; 94.8% of the 39ArK; MSWD=4.7). Radiogenic yields for sample 3-88 are very low and in some cases are negative (over corrected for extraction line blank). K/Ca ratios range from 1.0 to less than 0.1. The inverse isochron also yields very imprecise results (age=0.32±0.35 Ma; 40Ar/36Ar intercept=296±16; MSWD=3.8). The age spectrum for the BG-4 86-3 hornblende (Figure S18) is very similar to that of the 19141-1 hornblende. Steps G through I yield a weighted mean age of 6.67±0.08 Ma with 95.8% of the 39ArK released and a MSWD of 0.2. Radiogenic yields for steps G through I range from 52 to 86% while the K/Ca ratios are relatively constant at approximately 0.1. The inverse isochron yields an age of 6.67±0.08 Ma with a 40Ar/36Ar intercept of 295±4 and a MSWD of 0.7. Discussion For the vast majority of samples dated in this report, the weighted mean or plateau age is interpreted to represent the age of eruption of the rock/mineral in question. The one exception to this is groundmass concentrate sample 19-133-1, where the inverse isochron yields the preferred age (32.45±0.32 Ma). The first two heating steps for 19-133-1 are significantly older than the remainder of the age spectrum. These anomalously old ages are likely caused by either small amounts of xenocrystic contamination or excess argon. Excess argon is non-atmospheric 40Ar within a sample that is derived by a process other than the in situ radioactive decay of 40K (McDougall and Harrison, 1999). Most commonly, excess argon refers to trapped 40Ar/36Ar compositions greater than 295.5 (the present day 40Ar/36Ar composition). In the case of the 19-133-1 groundmass concentrate, small amounts of excess argon may have been incorporated into glass and/or mineral phases at elevated argon partial pressures (i.e. at depth or in a magma chamber). In many cases, an inverse isochron is employed to test for trapped 40Ar/36Ar compositions greater than 295.5. The inverse isochron for the 19-133-1 groundmass concentrate yields a trapped 40Ar/36Ar composition (300±2) only slightly greater than 295.5. The slightly elevated 40Ar/36Ar trapped component prevents an unequivocal assessment of the scale of excess argon contamination, if it is present at all. However, given that the isochron data yields in an acceptable MSWD (1.8) while the plateau does not (3.3), we conclude that the inverse isochron age (32.45±0.32 Ma) is the best estimate for the age of eruption for this sample. The age spectra for groundmass concentrate sample 19-135-1 and, to a lesser extent, 19-137-1 suggest minor amounts of 40Ar loss. The anomalously young ages for the initial heating steps, coupled with the very low radiogenic yields for those steps, indicate the presence of alteration/hydration products. Despite the existence of this alteration, the majority of the age spectrum for both 19-135-1 and 19-137-1 does not appear to be greatly influenced. Therefore, we interpret the weighted mean ages for 19-135-1 (32.74±0.58 Ma) and 19-137-1 (8.72±0.11 Ma) as the preferred age of these samples. The 19-141-2B exhibits the characteristic age spectrum shape often attributed to excess argon. Assuming that the oldest ages in 19-141-2B are those most influenced by excess argon and that the youngest ages are those least influenced by excess argon, it is desirable to assign an age to a sample based solely on the youngest age. In the case of 19-141-2B, the youngest age is yielded by the weighted mean of steps D through F (7.26±0.10 Ma). However, it is difficult to ascertain the quantity of excess argon that may or may not be present in steps D, E and F, especially given the poor quality of the inverse isochron results. Therefore, while we state that the weighted mean age for this sample is the preferred age, we must also state that this age should be considered a maximum age. The presumed contamination by excess argon is supported by a biotite mineral separate from the 19-1412B sample (see below). The 19-141-2B groundmass concentrate yields a plateau weighted mean age approximately 350,000 years older than the 19-141-2B biotite plateau weighted mean age. Given that there are no signs of argon loss (e.g. alteration) in the biotite age spectrum, the source of the age discrepancy is undoubtedly the presence of excess argon in the 19-141-2B groundmass concentrate. Excess argon also appears to be influencing the higher temperature heating steps of the 20-492 groundmass concentrate age spectrum. However, unlike the 19-141-2B age spectrum, the lower temperature steps of the 20-49-2 age spectrum do not appear to be significantly influenced by excess argon. Therefore, the preferred age of 20-49-2 is the plateau weighted mean age of 0.33±0.05 Ma. Groundmass concentrate sample 3-89 does not appear to be influenced by either excess argon or alteration products (as indicated by both the shape of the age spectrum and the close agreement between the plateau weighted mean age and the inverse isochron age). Therefore, the preferred age of this sample is the plateau weighted mean age of 0.25±0.02 Ma. For each of the five biotite samples in this study, the plateau weighted mean age yields the best estimate for the age of eruption for each respective sample. Biotite Sample Age ± Error (2s) 19-141-2A 6.87±0.08 Ma 19-141-2B 6.89±0.05 Ma 20-49-3 0.51±0.03 Ma BG-4 84-2A 7.06±0.08 Ma BG-4 84-2B 7.31±0.07 Ma Only two biotite age spectra suggest any anomalous behavior: BG-4 84-2A and BG-4 84-2B. Both of these age spectra yield anomalously young ages in the lowest temperature heating steps. Like groundmass concentrate samples 19-135-1 and 19-137-1, biotites BG-4 84-2A and BG-4 84-2B may have small amounts of alteration products (chlorite?) present. However, despite the presumed presence of some alteration products, any 40Ar loss seems to be confined to the lowest temperature portions of the samples. Therefore, the plateau weighed mean ages are still the preferred ages for these samples. Although the hornblende samples were degassed in only 2 or 3 heating steps (as opposed to the 5 to 9 steps for most of the groundmass concentrate and biotite samples from this study) and despite some minor discordance with their age spectra, we interpret the hornblende plateau weighted mean ages as being the best estimates of their age of eruption. Hornblende Sample Age ± Error (2s) 19-57-2 1.39±0.21 Ma 19-141-1 6.71±0.08 Ma 3-87 2.85±0.83 Ma 3-88 0.34±0.32 Ma BG-4 86-3 6.67±0.08 Ma Many of the disturbances observed in the hornblende age spectra can be attributed to excess argon. For example, in sample 19-57-2, the age spike at the beginning of the age spectrum is probably the result of mineral inclusions or groundmass exposed on the surface of the hornblende separate, as suggested by the increase in K/Ca ratio for those steps. The same can also be said for the low temperature steps for hornblende sample 3-87. The highest temperature steps (J and K) for 3-87 may also be caused by mineral inclusions. In the case of hornblende 3-87, where a plateau is straddled by higher ages (similar to groundmass 19-141-2B) it is difficult to assess how much influence the excess argon is imparting on the plateau age. Therefore, like 19-141-2B, we must conclude that the age of 387 is a maximum age. Figure S18 shows an age summary plot for the sixteen samples dated in this study. The most striking feature of the data is the apparent grouping of samples into three distinct pulses of magmatism. The earliest phase determined from this sample suite is at approximately 32.5 Ma, as recorded by samples 19-133-1 and 19-135-1. Following the 32.5 Ma activity, about 25 million years passes before more magmatic activity is recorded at approximately 8.7 to 6.7 Ma, as recorded by seven samples (eight with the 19-141-2B duplicate). At least another 3 million years of no activity passes until the final pulse begins at approximately 2.8 Ma, as recorded by six samples. It must be noted, however, that sampling procedures as well as incomplete exposures can potentially result in an apparent cluster of magmatic activity. References Cande, S.C., and Kent, D.V., 1992. A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research, 97, 13,917-13,951. Channell, R., McMillan, N.J., Lawton, T.F., Heizler, M., Esser, R.P. and McLemore, V.T., 2000. Magmatic History of the Little Hatchet Mountains, Hidalgo and Grant Counties, Southwestern New Mexico. New Mexico Geological Society, Guidebook 51, p. 141-148.. Deino, A., and Potts, R., 1990. Single-Crystal 40Ar/39Ar dating of the Olorgesailie Formation, Southern Kenya Rift, J. Geophys. Res., 95, 8453-8470. Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977. Interpretation of discordant 40Ar/39Ar age-spectra of Mesozoic tholeiites from Antarctica, Geochim. Cosmochim. Acta, 41, 15-32. Giletti, B.J., 1974. Studies in Diffusion I: Argon in phlogopite mica. In Geochemical transport and kinetics (ed. A.W. Hofmann, B.J. Giletti, H.S. Yoder, Jr., and R.A. Yund), pp. 107-115. Carnegie Inst. Of Wash. Publ. 634. Harrison, T.M., 1981, Diffusion of 40Ar in hornblende: Contributions to Mineralogy & Petrology, v. 78, p. 324-331. Mahon, K.I., 1996. The New "York" regression: Application of an improved statistical method to geochemistry, International Geology Review, 38, 293-303. Samson, S.D., and, Alexander, E.C., Jr., 1987. Calibration of the interlaboratory 40Ar/39Ar dating standard, Mmhb-1, Chem. Geol., 66, 27-34. Steiger, R.H., and JŠger, E., 1977. Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planet. Sci. Lett., 36, 359-362. Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p. McDougall, I., and T. M. Harrison, 1988, Geochronology and thermochronology by the 40Ar/39Ar method: Oxford Monographs on Geology and Geophysics, v. 9, p. 212. York, D., 1969. Least squares fitting of a straight line with correlated errors, Earth and Planet. Sci. Lett., 5, 320-324. 2006tc002091-ts01.txt: Table S1. A 40Ar/39Ar summary table and analytical procedures. 2006tc002091-ts02.xls, 2006tc002091-ts02.txt: Table S2. The 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples. Figure Captions 2006tc002091-fs01.eps: Figure S1. Generalized stratigraphy of the Alborz showing formations, approximate thickness, and lateral continuity. Colors correspond to Figure 2; star indicates units identified as clasts in Oligocene-Miocene conglomerates. 2006tc002091-fs02.eps: Figure S2. The 40Ar/39Ar age spectrum and inverse isochron for the 19-133-1 groundmass concentrate. 2006tc002091-fs03.eps: Figure S3. The 40Ar/39Ar age spectrum and inverse isochron for the 19-135-1 groundmass concentrate. 2006tc002091-fs04.eps: Figure S4. The 40Ar/39Ar age spectrum and inverse isochron for the 19-137-1 groundmass concentrate. 2006tc002091-fs05.eps: Figure S5. The 40Ar/39Ar age spectrum and inverse isochron for the 19-141-2B groundmass concentrate. 2006tc002091-fs06.eps: Figure S6. The 40Ar/39Ar age spectrum and inverse isochron for the 20-49-2 groundmass concentrate. 2006tc002091-fs07.eps: Figure S7. The 40Ar/39Ar age spectrum and inverse isochron for the 3-89 groundmass concentrate. 2006tc002091-fs08.eps: Figure S8. The 40Ar/39Ar age spectrum and inverse isochron for the 19-141-2A biotite. 2006tc002091-fs09.eps: Figure S9. The 40Ar/39Ar age spectrum and inverse isochron for the 19-141-2B biotite. 2006tc002091-fs10.eps: Figure S10. The 40Ar/39Ar age spectrum and inverse isochron for the 20-49-3 biotite. 2006tc002091-fs11.eps: Figure S11. The 40Ar/39Ar age spectrum and inverse isochron for the BG-484-2A biotite. 2006tc002091-fs12.eps: Figure S12. The 40Ar/39Ar age spectrum and inverse isochron for the BG-484-2B biotite. 2006tc002091-fs13.eps: Figure S13. The 40Ar/39Ar age spectrum and inverse isochron for the 19-57 hornblende. 2006tc002091-fs14.eps: Figure S14. The 40Ar/39Ar age spectrum and inverse isochron for the 19-141-1 hornblende. 2006tc002091-fs15.eps: Figure S15. The 40Ar/39Ar age spectrum and inverse isochron for the 3-87 hornblende. 2006tc002091-fs16.eps: Figure S16. The 40Ar/39Ar age spectrum and inverse isochron for the 3-88 hornblende. 2006tc002091-fs17.eps: Figure S17. The 40Ar/39Ar age spectrum and inverse isochron for the BG-486-3 hornblende. 2006tc002091-fs18.eps: Figure S18. Summary of the 40Ar/39Ar ages yielded by the samples from Alborz Mountains, Iran.