Exhumational history Tajikistancommunity.middlebury.edu/~wamidon/pdfs/...inreview.pdf · 32 the Karakoram and Altyn Tagh/Karakax, both of which terminate in the eastern margin of

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Title 2

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Exhumational history of the Karakul graben, Tajikistan 4

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Authors 8

William H. Amidon1, Scott A. Hynek2 9

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Author Affiliations 12

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1. California Institute of Technology, GPS Division, MS 100-23, Pasadena, CA 91125 14 2. University of Utah, Department of Geology and Geophysics, 135 S. 1460 E, Salt Lake 15

City, UT 84112 16 17

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Corresponding Author 22

William H. Amidon 23

Email: [email protected] 24

Phone: 626‐395‐2190 25

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Abstract 28

The Pamir plateau forms a prominent tectonic indentation along the western end of the Tibet‐29

Tarim margin, where ~75 km thick crust has advanced as much as 300 km northward over the Eurasian 30

margin. Indentation of the Pamir is partially accommodated by two of Asia’s largest strike‐slip faults, 31

the Karakoram and Altyn Tagh/Karakax, both of which terminate in the eastern margin of the Pamir 32

indentor. Despite its tectonic significance, relatively little is known about the timing of major Cenozoic 33

tectonic events in the Pamir. We present new apatite and zircon (U/Th)‐He ages, bulk‐rock 34

geochemistry and Al‐in‐hornblende barometry results from the Karakul graben in the north‐central 35

Pamir. The Karakul graben is a prominent north‐south oriented rift basin sitting ~50 km south of the 36

Main Pamir Thrust, and ~150 km west of the Kongur Shan extensional system. Cooling ages and 37

thermobarometry results suggest that relatively slow exhumation throughout much of the late Mesozoic 38

and Cenozoic was punctuated by two periods of accelerated exhumation between ~55‐40 Ma, and 24‐39

16 Ma. We interpret the first period of rapid exhumation (~0.55 m/my) as a result of accelerated 40

tectonic uplift and subsequent erosion due to the northward propagation of the India‐Asia collision. 41

Based on the pattern of cooling ages, we attribute the second period of rapid exhumation (0.6‐0.8 42

m/my) to the onset of extensional faulting in the Karakul graben. Early Miocene extension predates that 43

previously inferred for nearby extensional systems, and suggests that rifting may not have been caused 44

by radial thrusting or oroclinal bending of the Pamir margin. Instead, we suggest that extension may 45

have been coincident with early activity along the Karkoram and Altyn Tagh faults, both of which may 46

once have terminated near the Karakul graben. 47

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1. Introduction 55

The Tibetan plateau is a semi‐rigid block bound by active orogens along its boundaries with 56

Eurasia to the north and with the Indian subcontinent to the south. Along the southern margin, India‐57

Asia convergence is accommodated by the thrust sheets of the Himalayan chain, and to the north by a 58

more disparate series of mountain belts including the Qilian Shan, Kunlun Shan, Tian Shan, and Pamir. 59

Although the onset of India‐Asia collision is well documented at ~55‐50 Ma, many questions remain 60

regarding how and when tectonic deformation propagated northward, thickening the Tibetan Plateau 61

and driving orogenesis along its northern edge [Hodges, 2000; Yin and Harrison, 2000]. 62

The Pamir mountains lie along the western end of the Tibet‐Tarim margin, where ~ 20 mm/yr of 63

India‐Asia convergence is accommodated across a relatively narrow zone of north‐vergent thrust 64

faulting [Arrowsmith and Strecker, 1999; Reigber et al., 2001; Strecker et al., 2003]. In map view, the 65

Pamir create a prominent oroclinal arc, mimicking the shape of the western Himalayan syntaxis 66

immediately to its south. A variety of ideas have been put forward to understand how this tectonic 67

indentation has evolved and its implications for deformation of the western Tibetan plateau. Early 68

observations suggested that large offsets on the Karakoram fault transferred strain to E‐W oriented 69

thrust faults within the Pamir [Burtman and Molnar, 1993; Peltzer and Tapponnier, 1988]. More recent 70

evidence suggests that total offsets along the Karakoram fault are moderate, and that the Pamir 71

indentation is caused by oroclinal bending accommodated by strike‐slip faulting along terrane 72

boundaries, vertical block rotation, and extensional faulting within terranes [Raterman et al., 2007; 73

Robinson, 2009; Robinson et al., 2007; Schwab et al., 2004] . Answering these questions requires a basic 74

chronology of when extensional faulting began in relation to strike‐slip faulting and crustal shortening in 75

within the Pamir. 76

In this paper, we discuss the thermal and tectonic history of the Karakul basin, one of the most 77

prominent physiographic features of the northern Pamir. Understanding the history of this relatively 78

simple extensional feature provides clues to the regional tectonic context at the time of its formation. 79

Early work proposed that the basin was an impact structure, citing shock quartz on the central island 80

[Gurov et al., 1993]. More recent studies, including this one, suggest that the basin is actually an 81

extensional rift basin that is currently experiencing NW‐SE directed transtensional deformation [Blisniuk 82

and Strecker, 1997; Strecker et al., 1995]. We present new apatite and zircon (U/Th)‐He cooling ages 83

from the Karakul basin in an effort to document the timing of extension. Our results show relatively 84

slow cooling since ~220 Ma punctuated by two episodes of rapid cooling and exhumation between 85

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~55‐40 Ma and ~24‐16 Ma. We interpret the first period of rapid exhumation as an erosional response 86

to tectonic uplift associated with the early stages of the India‐Asia collision. We discuss the second 87

period of rapid exhumation using two end‐member hypotheses: 1) cooling ages record rapid erosion in 88

response to continued tectonic thickening above the Main Pamir Thrust, or 2) that the rapid cooling 89

beginning at ~24 Ma records the onset of extensional faulting. We favor the latter hypothesis and 90

propose a mechanism by which early slip along the Altyn Tagh and Karakoram strike‐slip faults 91

accommodates extension in the Karakul basin. 92

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2. Geological Setting 94

The Pamir mountains are composed of Paleozoic‐ Mesozoic accreted terranes that are deflected 95

northward from their continuations across Afghanistan and Tibet [Yin and Harrison, 2000]. Although 96

correlation of these terranes is much debated, we adopt the idea that the Northern Pamir terrane 97

correlates with the Songpan‐Garze terrane, and the Southern Pamir with the Qiantang terrane 98

[Robinson, 2009; Schwab et al., 2004; Yin and Harrison, 2000]. This correlation is most convincingly 99

supported by the data of Schwab et al. (2004), who identify a continuous chain of ~200 Ma plutons with 100

similar geochemical characteristics wrapping around the Pamir from the Karakul Basin, through the Muji 101

basin and into the Mazar region of the western Kunlun (Figure 1). Based on bulk‐rock geochemistry, 102

they divide these late Jurassic plutons into the Kunlun magmatic arc (north of the Kunlun suture), and 103

the Karakul‐Mazar belt (south of the Kunlun suture). We present additional geochemical data 104

supporting the correlation of the Karakul and Mazar granites, suggesting that the Karakul graben sits 105

south of the Kunlun suture. 106

Late Cenozoic tectonics of the eastern Pamir (Figure 1) are dominated by: 1) northward 107

thrusting of the Pamir ‐Alai range over the Eurasian margin along the Main Pamir Thrust (MPT), 2) east‐108

west extension along the Kongur Shan extensional systems, and 3) strike‐slip motion along the 109

westernmost extensions of the Karakoram and Altyn Tagh/Karakax systems. The MPT is a complex 110

structural feature, which originated as a Paleozoic suture, and has likely experienced several episodes of 111

shortening since that time. Observations from deep exposures in the Kongur Shan region document the 112

growth of ramp‐thrust anticlines above the MPT during the Cenozoic, with a large increase in the growth 113

of metamorphic garnet beginning at ~45 Ma [Robinson et al., 2007]. In contrast, the sedimentary 114

record in the Alai valley suggests N‐S shortening was broadly distributed during the Eocene and 115

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Oligocene before becoming focused along the modern MPT around middle Miocene time [Coutand et 116

al., 2002]. Uplift and exhumation appear to have intensified along the western part of the MPT during 117

the early Miocene as recorded by apatite fission track cooling ages near the Kumtag thrust [Sobel and 118

Dumitru, 1997]. 119

Northward advancement of the Pamir salient was partially accommodated by right‐lateral slip 120

along the Karakoram fault system which runs >1000 km from northern Pakistan. Estimates for the 121

timing of initiation along the southern part of the fault range from <13 Ma [Murphy et al., 2000], to 13‐122

15 Ma [Phillips et al., 2004], to 25‐22 Ma, or possibly older [Lacassin et al., 2004; Valli et al., 2008]. At 123

its northern termination the Karakoram fault appears linked to a horsetail pattern of strike‐slip faults, 124

some linked with thrust faults in the Rushan‐Pshart zone [Burtman and Molnar, 1993; Searle, 1996; 125

Strecker et al., 1995]. Offsets of the Aghil limestone suggest a total displacement of ~160 km along the 126

northern section of the Karakoram [Robinson, 2009]. A second major strike‐slip fault, the Karakax fault, 127

extends westward from the Altyn Tagh system before turning northward and running sub‐parallel to the 128

Karakoram fault along the eastern flank of the Pamir salient. The Altyn Tagh/Karakax system has 129

accumulated > 400 km of total offset [E. Cowgill et al., 2003; Ritts and Biffi, 2000; Yin and Harrison, 130

2000]. Paleomagnetic data from sediments along the Altyn Tagh suggest initiation ~24 Ma based on 131

little observed rotation prior to that time [Chen et al., 2002]. If Quaternary slip rates of ~10‐25 mm/yr 132

are extrapolated over the lifetime of the fault, they suggest initiation sometime between ~15‐45 Ma [E. 133

Cowgill et al., 2003; E. Cowgill et al., 2009; Meriaux et al., 2004]. 134

The Karakul graben is often discussed in the context of three other normal fault systems (Muji, 135

Kongur Shan, and Tashkurgan) that collectively accommodate extension along the northern and eastern 136

margins of the Pamir plateau. The northernmost Muji graben appears to be oblique, and perhaps linked 137

to the northern edge of the Karakul graben by way of the right‐lateral Markansu fault [Strecker et al., 138

1995]. The Kongur Shan system is primarily oriented north‐south, making an arcuate bend where it 139

bounds the Mustagh‐Ata massif and terminating near the northern end of the Tashkurgan valley 140

[Robinson et al., 2007]. The Karakul graben may also be linked to the Kongur Shan and Tashkurgan 141

systems by the right lateral Rangkul strike‐slip fault. The onset of extension along the Kongur Shan 142

system has been inferred to be ~7‐8 Ma based on 40Ar/39Ar cooling ages and on the youngest ages of 143

matrix‐sited monazite [Robinson et al., 2004; 2007]. Extension has proceeded rapidly since the late 144

Miocene, producing 40Ar/39Ar muscovite ages as young as 2 Ma [Arnaud et al., 1993], and accumulating 145

a maximum of 35 km of east‐west extension which decreases to the south [Brunel et al., 1994; Robinson 146

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et al., 2004; 2007]. Several ideas have been suggested to explain extension around Kongur Shan and 147

Karakul including; orogenic collapse due to gradients in crustal thickness, radial thrusting and/or 148

oroclinal bending of the Pamir, and transfer of strain from the Karakoram fault [Ratschbacher et al., 149

1994; Robinson et al., 2004; Strecker et al., 1995; Yin et al., 2001]. 150

The Karakul graben itself is a ~50 km wide depression bounded by steep north‐south oriented 151

flanks with a total relief of >1500 m. The graben sits atop ~75 km thick crust, whose uppermost section 152

is composed of strongly folded and faulted Paleozoic meta‐sediments intruded by latest Triassic 153

granodiorites [Romanko, 1968; Schwab et al., 2004]. Near the northern and southern ends of the lake, 154

prominent fault scarps and earthquake focal mechanisms reveal oblique slip vectors, suggesting that the 155

lake is currently undergoing a trans‐tensional stress regime [Blisniuk and Strecker, 1997; Strecker et al., 156

1995]. At its widest point, the eastern flank of the graben does not exhibit active normal fault scarps, 157

although its linear shape, steep relief, and sediment‐laden footwall are indicative of normal faulting. 158

The primary evidence for normal faulting along the western flank is derived from recent fault scarps to 159

the north and south, and from the steep bedrock slopes plunging directly into the lake. The steep linear 160

flanks of the central island suggest a horst separating two grabens, although there is no direct evidence 161

for normal faulting. The only existing thermochronology results from the Karakul basin are apatite 162

fission‐track ages ranging from ~24‐17 Ma, which are mentioned in an abstract [Schwab et al., 1999]. 163

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3. Methods 165

Samples were collected from four vertical transects of granodiorites containing plagioclase > 166

quartz >biotite ± K‐feldspar ± muscovite, with abundant zircon, apatite, epidote and titanite occurring as 167

accessory phases. The petrology of the NE profile differed significantly from the others, and is 168

presented in online table 3. Sample TAJ93 from the NE profile is a hornblende‐diorite, whereas samples 169

TAJ96 and TAJ102 are garnet bearing granodiorites with > 1 cm sized garnets in sample TAJ96. 170

Inclusion‐free apatites and zircons were hand‐picked, and measured in all three dimensions. Alpha 171

ejection correction factors (Farley et al. 1996) were computed from length and mean width. Apatites 172

were analyzed for He, U, and Th following published procedures [House et al., 2000]. Zircons were 173

degassed for helium analysis by lasing inside of sapphire micro‐furnaces with removable cover pieces for 174

easy removal of grains following lasing. The micro‐furnaces were heated to ~1510 °C for 12 minutes, 175

and helium re‐extracts typically yielded <1% of the primary extraction. Helium blanks were consistently 176

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less than 0.1% of sample concentrations for apatite, and even less for zircon. Zircons were then 177

transferred into parr bomb capsules and spiked for measurement of U and Th by isotope dilution. Parr 178

bomb dissolution proceeded by dissolving samples in HF at 220 °C for 48 hours, drying down and 179

redissolving in 6 N HCL for 24 hours, followed by a final dry‐down and redissoluton in a 5% nitric acid 180

solution for ICP‐MS analysis. Blanks achieved by this method were typically ~3 pg of U and ~20 pg of Th, 181

representing < 0.1% of measured U and <3% of measured Th in all samples. Seven replicate analyses, 182

each consisting of multiple Durango apatite grains yielded a mean age of 32.5 ± 1 My [K. A. Farley, 2000; 183

Young et al., 1969]. Three replicate analyses of single zircon grains from the Fish Canyon tuff yielded a 184

mean age of 27.7 ± 1.5 My [P. W. Reiners et al., 2002; Renne et al., 1994]. Complete datasets are 185

available in online tables 1 and 2. 186

Compositional analyses on individual mineral phases were made on polished rock sections using 187

the JEOL JXA‐8200 electron microprobe at Caltech. Bulk rock geochemical analyses were made at 188

Michigan State University on samples from the three different profiles, as well as a fourth sample from a 189

basaltic dike (sample P916‐1). For each profile, a fist‐sized piece of rock was taken from each sample in 190

the profile, amalgamated by powdering and then fused into a single glass disk. Major element analyses 191

were made using a Bruker S4 Pioneer XRF instrument, whereas trace elements and REE’s were analyzed 192

by LA‐ICP‐MS using a 266 nm Nd:YAG laser coupled to a Micromass hexapole ICP‐MS. 193

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4. Results 195

Single grain apatite and zircon (U/Th)‐He ages are presented from all four vertical transects, and 196

from three additional sites near lake level (Table 1; online tables 1 and 2). Ages are calculated 197

following the standard age equation, including decay of Sm for apatite, but not for zircon. Ages are 198

adjusted for alpha‐ejection using FT factors calculated following Farley et al. (1996), which ranged from 199

0.77‐0.85 for apatite and 0.8‐0.9 for zircon. Because only ~3 individual grain analyses were performed 200

per sample, it is difficult to assign an external reproducibility for each sample. Instead, we determine a 201

single standard deviation for the entire population by normalizing each analysis by its mean sample age. 202

This procedure gives ~ 7% standard deviation for both apatite and zircon analyses, which we adopt as 203

the uncertainty on all ages. One major exception are six apatite analyses from sample P918‐1, which 204

reproduce very poorly and are not included in the population‐wide standard deviation. Interestingly, 205

this is the only sample that exhibits a shear fabric, suggesting that the poor reproducibility could be due 206

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to thermal disturbance, micro‐inclusions, or lattice damage acquired during shearing. Two additional 207

analyses P919‐3a (apatite) and P916‐4c (zircon) are also assumed to be outliers and are included in the 208

population‐wide standard deviation, but not in their respective sample means. 209

Our youngest apatite ages cluster between ~19‐15 Ma, and are obtained on the two northern 210

profiles, as well as the bottom of the SE profile. All apatite ages on the two northern profiles are within 211

2σ error of the other ages from the same profile. Another group of apatite ages cluster between ~44‐33 212

Ma, observed in samples from the southern part of the lake. Zircon ages can be divided into three 213

groups: 1) 24‐21 Ma ages along the northwestern profile, 2) 40‐38 Ma ages along the northeastern 214

profile, and 3) mostly ~165‐150 Ma ages in the southern part of the lake. All zircon ages along the 215

northwestern profile are also within 2σ error of each other. 216

Mineral compositions from the NE transect (Online Table 3) were analyzed in an effort to 217

estimate the pressure and temperature conditions of pluton emplacement. Compositional profiles 218

across garnet‐biotite grain boundaries show highly elevated Mn and depleted Mg concentrations near 219

the rims, suggesting that the garnets were altered during retrograde diffusion, or were incorporated 220

from mafic schists in the wall rock [Kohn and Spear, 2000]. Garnet‐biotite temperature estimates for 221

samples TAJ93 and TAJ102 do not converge on reasonable results and are not discussed further. Sample 222

TAJ93 contains abundant hornblende, as well as a K‐feldspar, plagioclase, ilmenite, titanite and epidote 223

mineral assemblage, ideal for Al‐in‐hornblende barometry [Hammarstrom and Zen, 1986; Hollister et al., 224

1987]. The cores regions of ~120 different hornblende grains were analyzed and a plot was made of Al 225

vs. Fe/(Fe+Mg) to assess the possibility of low‐temperature alteration. This plot shows a positive linear 226

relationship of shallow slope indicating a small amount of alteration in some grains. To best 227

approximate an unaltered composition, we base our Al‐in‐Hornblende pressure calculations on the 228

average composition of the quartile of analyses containing the highest Al contents and Fe/(Fe+Mg) 229

ratios. Mineral compositions are presented in online Table 3 and yield emplacement pressures of ~2.3 ± 230

0.5 Kb using the Probe‐Amph program [Tindle and Webb, 1994]. 231

Bulk rock geochemistry analyses were also performed to provide a comparison between the 232

Karakul granodiorites and their probable counterparts in the western Kunlun region. Geochemistry 233

results are presented in figure 4 and in online table 4 and show that samples P916, P917, and P919 all 234

have typical granodiorite compositions, whereas the mafic dike from the SW transect (sample P916‐1) 235

has an alkaline basalt composition. The total REE content of the granodiorites is quite low (~150‐200 236

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ppm), characterized by moderate LREE enrichment (LREE/HREE= 3.5‐6.5) and a weak Eu anomaly. 237

Comparison of our data with plutons from the north and south Kunlun belts [Yuan et al., 2002; Yuquan 238

et al., 1996], as well as that of Schwab et al. (2004), supports the idea that the Karakul granites are 239

correlative with the Mazar granites, and are slightly different from granites of similar age north of the 240

Kunlun suture. The REE pattern from the Karakul granites is virtually identical to the Mazar granodiorite, 241

but has notably lower concentrations of HREE’s than the Arkanz Intrusive Complex (ARIC) that occurs 242

north of the Kunlun suture near Kudi [Yuan et al., 2002]. Trace element data also support this 243

correlation, with the Mazar granodiorite providing an almost identical match to the Karakul granites. 244

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5. Interpretation of Thermal History 246

In the following discussion, we use (U/Th)‐He ages in two ways: 1) we compare pairs of zircon 247

and apatite ages from the same sample, and 2) we compare age‐elevation plots (vertical transects) of 248

apatite or zircon ages. The goal of the first approach is to exploit the difference in closure temperature 249

between zircon (~180 °C) and apatite (~65 °C) to understand how long it took for a given sample to cool 250

between these two temperatures [K.A. Farley, 2002; P.W. Reiners et al., 2004]. However, because 251

different thermal histories can produce identical ages in a given crystal, it is necessary to interpret these 252

paired thermochronometers using a time‐temperature‐age model such as the HeFTy program [Ketcham, 253

2005]. When using HeFTy, we adopt the He diffusion kinetics of Farley et al. (2000) for apatite and 254

those of Reiners et al. (2004) for zircon. In the second approach (vertical transects), samples are 255

collected along the steepest possible slopes (i.e. fault scarps), under the assumption that these 256

approximate a vertical slice into a laterally homogenous crust. The slope of a line in age versus elevation 257

space can then be used to directly infer the vertical exhumation rate if it is assumed that: 1) the closure 258

temperature was the same for all samples in the transect, 2) the thermal gradient was constant during 259

the period considered, and 3) the block has not been significantly tilted [Ehlers and Farley, 2003; Stockli 260

et al., 2003]. To compare the exhumation rates inferred from vertical transects with cooling rates from 261

our paired apatite and zircon ages, we assume a thermal gradient of 25 °C/Km. 262

Several lines of evidence suggest that the Karakul granites cooled quickly following their 263

emplacement in the latest Triassic, and then continued to cool very slowly during the late Jurassic and 264

Cretaceous periods. For example, the short time between zircon crystallization ages (~220 Ma) and the 265

biotite 40Ar/39Ar cooling ages (~203Ma) in the southern part of the lake, suggests that the Karakul 266

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granites were emplaced at relatively shallow depths and cooled quickly through ~300°C [Schwab et al., 267

2004]. This is consistent with our Al‐in‐hornblende barometry results which suggest that sample TAJ93 268

was emplaced at depths of 7‐9 km (~1.9‐2.5 Kb). The ~160 Ma zircon (U/Th)‐He ages in the southern 269

part of the lake imply that rapid cooling (~6 °C/Myr) continued between ~203‐160 Ma, as the plutons 270

came into equilibrium with the local thermal gradient. Based on the difference between (U/Th)‐He ages 271

in co‐existing apatite and zircon in samples from the southern transects, the Karakul granites then 272

experienced a long period of extremely slow cooling ( ~0.9‐1.0 °C/My) and exhumation (0.035‐0.04 273

mm/yr) between ~160‐55 Ma. 274

Our data suggest that the rate of cooling increased significantly sometime around ~55‐45 Ma. 275

One piece of evidence for this interpretation is the paired zircon (~165‐150 Ma) and apatite (~45‐38 Ma) 276

ages from rocks in the southern part of the lake. Taken alone, these paired ages (samples p917‐1, P916‐277

1, p916‐3, and p916‐4) could be explained by a constant cooling rate of ~0.7 °C/Myr beginning around 278

160 Ma (after closure of zircon) and continuing until today. However, this solution is unlikely based on 279

the additional constraint provided by the zircon ages in the NW transect, which indicate a rapid 280

exhumation rate of ~0.55 ± 0.2 mm/yr (1σ), or a cooling rate of ~14 ± 5 °C/yr at 40‐38 Ma. If we assume 281

that this reflects the regional cooling rate at this time, and therefore apply it to samples from the 282

southern part of the lake, it requires a thermal history in which there is a period of very slow cooling (< 283

0.5 °C/yr) until at least 55 Ma, followed by a shift to more rapid cooling. Applying this cooling rate over 284

30‐40 km spatial scales is reasonable for two reasons: 1) when extension is restored, the profiles will be 285

much closer together, and 2) because it reflects cooling of samples at 6‐9 km depth, where the effect of 286

differential surface erosion rate on cooling at depth is greatly damped [P. W. Reiners, 2007]. An 287

acceleration in cooling and exhumation rates at ~55‐45 Ma is consistent with the possible break in slope 288

observed in the SW apatite He age profile, suggesting that the highest sample in the SW profile may be 289

the bottom of an exhumed partial retention zone (Figure 3). 290

Sometime after ~40 Ma the average rate of exhumation must have slowed throughout the 291

region. For example, if exhumation had continued from 40 Ma onward at the rate of 0.55 mm/yr, or 292

even at the 1σ lower bound of 0.25 mm/yr, we would expect < 50 Ma zircon ages to have been 293

exhumed in the southern part of the basin. Instead, the youngest observed zircon age is ~135 Ma. 294

Exhumation also appears to have slowed to a lesser degree along the NW profile sometime after 40 Ma. 295

Time‐temperature age modeling using the HeFTy program indicates that if exhumation had been 296

proceeding at 0.55 mm/yr at 40 Ma, it would need to slow to a linear rate of ~0.05 mm/yr shortly 297

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thereafter to produce the 20‐18 Ma apatite cooling ages observed in the same samples. If it had been 298

proceeding at the 1σ lower rate of 0.25 mm/yr, it would need to slow slightly to ~0.2 mm/yr. Likewise, 299

the rate of exhumation since ~18 Ma cannot have exceeded ~0.15 mm/yr without bringing even 300

younger apatite cooling ages to the surface at the base of the NW and NE transects. 301

The overall decrease in cooling rate after ~40 Ma rate conflicts with the steep age‐elevation 302

relationships observed in the NE and NW transects between ~24‐16 Ma. For example, apatite ages 303

from the NW transect imply an exhumation rate of > 0.5 mm/yr from ~20‐18 Ma. The apatite age‐304

elevation transect from the NE profile indicates an exhumation rate >0.6 mm/yr between 19‐17 Ma. 305

Additionally, time‐temperature age modeling of paired apatite and zircon cooling ages in sample TAJ99 306

and TAJ102 from the NE transect suggest that exhumation rates reached ~0.8 mm/yr between ~22‐15 307

Ma [Ketcham, 2005]. The simplest thermal history that satisfies all of the above constraints is one in 308

which a relatively slow Cenozoic cooling rate is punctuated by two periods of rapid exhumation at ~55‐309

45 Ma and ~24‐16 Ma. 310

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6. Interpretation of Tectonic History 312

The data presented above argue that the Karakul basin experienced two episodes of accelerated 313

exhumation between ~55‐45 Ma and ~24‐16 Ma. We attribute the first episode of cooling to 314

accelerated tectonic uplift and erosion in response to India‐Asia collision. As discussed below, we 315

attribute the second episode of cooling to extensional faulting within the Karakul graben. Because of 316

its proximity, we had expected (U/Th)‐He ages from the Karakul graben to record the onset of 317

extensional faulting at a similar time to the Kongur Shan system, around 7‐8 Ma [Robinson et al., 2004]. 318

The observation of considerably older cooling ages raises the question of whether rapid exhumation 319

recorded at ~24‐16 Ma reflects: 1) regional tectonic uplift and erosion followed by a small amount of 320

post‐16 Ma extensional faulting, or 2) tectonic exhumation due to the onset of extensional faulting at 321

~24 Ma. We favor the latter hypothesis for the reasons discussed below. 322

In the first interpretation, the Karakul region would have experienced uplift and erosion as part 323

of a hinterland plateau structurally above and behind the Main Pamir Thrust (MPT) front. Rocks at the 324

surface today would have been exhumed through their closure isotherm during the early Miocene and 325

then remained at depths of < 3 km until being exhumed by a small amount of extensional faulting 326

sometime after ~16 Ma. This interpretation is unlikely because even a small amount of post‐16 Ma 327

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extension should have exhumed some rocks from the top of the apatite He partial retention zone with 328

ages younger than ~16 Ma. For example, assuming a geothermal gradient of 25 °C/km and 65°C closure 329

temperature, a 60° dipping normal fault would require ~1.5 km of horizontal extension to exhume 330

apatite U/Th‐He ages from the top of the partial retention zone, whereas a 45° fault would require ~2.5 331

km. Although we are unable to make estimates of the amount of extension, the graben is ~50 km wide 332

between the crests of the east and west flanks, suggesting it has experienced more than 2.4 km of 333

extension on one or both of its flanks. 334

A second argument against a post‐ 16 Ma onset of extension is the asymmetry in cooling ages 335

observed between the NW and NE transects. Coexisting apatite and zircon cooling ages in the NE profile 336

span ~24‐16 Ma, whereas coexisting ages in the NW transect span ~40‐18 Ma, indicating that a larger 337

amount of exhumation occurred along the NW transect during the early Miocene. Although such a 338

pattern could be created by fluvial incision and subsequent topographic inversion, it is more easily 339

explained by a larger magnitude of slip on the NE basin bounding normal fault, beginning at ~24‐22 Ma. 340

This interpretation is consistent with the mafic mineral assemblages of rocks from the NE transect, 341

which may indicate a lower level of the magma chamber. 342

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7. Discussion 344

The simplest explanation of the observed pattern of cooling ages is that extension in the Karakul 345

graben began in the early Miocene, at around 24‐22 Ma. Early Miocene initiation of extensional 346

faulting in the Karakul graben would significantly predate the 7‐9 Ma onset of extension inferred for the 347

Kongur Shan system [Robinson et al., 2004; 2007]. However, this age is based primarily on ubiquitous 7‐348

9 Ma 40Ar/39Ar cooling ages, and a large population of 8‐10 Ma monazite ages found in the matrix of 349

deformed schists and gneisses. An alternative explanation is that the onset of extension occurred 350

simultaneously between the Kongur Shan and Karakul systems at ~24‐22 Ma before the Karakul graben 351

was abandoned and extension became focused entirely along the Kongur Shan system during the late 352

Miocene and Pliocene. This idea can be reconciled with the data of Robinson et al. (2007) if it is 353

assumed that the end of prograde metamorphism and the onset of extensional collapse is represented 354

by the youngest zircon ages (~22 Ma) observed in the footwall orthogneiss of the Kongur Shan normal 355

fault. It is also consistent with the youngest garnet‐hosted monazite Th‐Pb ages (~24 Ma) from the 356

footwall rocks in the Mustagh‐Ata area, and with the broad peak in garnet hosted monazite ages (~18‐357

13

28 Ma) in rocks south of Mustagh‐Ata. Such an interpretation requires that the abundance of 8‐10 Ma 358

matrix‐sited monazite ages be explained by recrystallization or dissolution/reprecipitation during uplift 359

and mylonitization [Robinson et al., 2004]. 360

Early Miocene extension in the Karakul graben has specific implications for the various tectonic 361

models that have been suggested to explain extensional faulting in the northeastern Pamir. An age of 362

24‐22 Ma likely predates accumulation of large offsets on the Karakoram fault, and significant 363

indentation of the Pamir salient [Raterman et al., 2007; Robinson, 2009]. This implies that the Karakul 364

graben did not initially form in response to radial thrusting or to oroclinal bending of the Pamir. Because 365

significant crustal thickening had already occurred above the MPT by the early Miocene [Robinson et al., 366

2004; Sobel and Dumitru, 1997], it is plausible that extension was driven by gravitational collapse due to 367

a steep gradient in crustal thickness between the Pamir plateau and the Tarim basin [Dewey, 1988; 368

Strecker et al., 1995]. 369

We propose a tectonic model building on previous suggestions that the Karakoram fault could 370

either be related to north‐trending rifts in Tibet or driven by strain transfer from the Karakoram fault 371

[Ratschbacher et al., 1994; Strecker et al., 1995]. We propose that initiation of extension in the Karakul 372

graben occurred near the western terminations of the Karakoram and Altyn Tagh faults, in response to 373

their co‐initiation ~24‐22 Ma. Although speculative, this idea is plausible for several reasons. First, 374

strike‐slip assisted normal faulting is a very common mode of extension throughout the Tibetan 375

plateau, and is required by some numerical models to induce orogen perpendicular extension in other 376

orogens [Armijo et al., 1989; Armijo et al., 1986; Robl et al., 2008; Selverstone, 2005]. Our idea is also 377

consistent with tectonic reconstructions of western Tibet that restore the northern Pamir terrane to the 378

Songpan‐Garze terrane in a linear east‐west orientation bounded along their northern edge by the 379

Kunlun Suture and Karakax‐Altyn Tagh fault system [E. Cowgill et al., 2003; Raterman et al., 2007]. Such 380

a correlation is further supported by the work of Schwab et al. (2004) and the additional geochemical 381

data that we present in section 4. Finally, onset of extension in the Karakul graben at ~24‐22 Ma 382

matches very well with some estimates of slip initiation along the Altyn Tagh and Karakoram faults 383

[Chen et al., 2002; Lacassin et al., 2004; Valli et al., 2008]. 384

A plausible evolution of this geometry is reconstructed schematically in figure 4, and includes 385

the following stages. In the late Oligocene and earliest Miocene we assume that the Altyn Tagh and 386

Karakax systems were a continuous system of left‐lateral strike‐slip faults acting to partition eastward 387

extrusion of Tibet from northward overthrusting over the Tarim basin [Raterman et al., 2007]. We 388

14

propose that the Karakax fault was linked to the Markansu fault on its eastern end at this time. Second, 389

we assume that initiation of the Karakoram fault occurred around 24‐22 Ma, with its northern tip 390

initially exploiting a pre‐existing system of east‐west trending left‐lateral strike‐slip faults, including the 391

proto Rangkul fault and perhaps parts of the Jinsha suture and/or Ghoza Co fault system. As slip on the 392

Karakoram and Altyn Tagh faults intensified, extensional faulting occurred on the Karakul and Kongur 393

Shan systems near the western terminations of the two faults. As the Pamir indentor subsequently 394

developed during the middle and late Miocene, the mechanical link between the Karakoram and 395

Rangkul faults was abandoned, extension ceased, and the Karakul graben was translated northward 396

riding atop rapidly thickening crust above the Main Pamir Thrust system. The preservation of the 397

graben for millions of years during northward translation is plausible in light of the fact that < 10 km of 398

total exhumation has taken place over the last 200 My. 399

After the Rangkul fault was abandoned, the northern tip of the Karakoram fault began 400

transferring strain to a succession of more southerly strike‐slip faults including the east‐Pamir 401

(Achiehkopai) fault [Robinson, 2009; Strecker et al., 1995], and currently to the Karasu strike‐slip system 402

[Amidon et al., 2008]. In contrast to the Karakul graben, extension likely accelerated along the Kongur 403

and Mustagh systems from the late Miocene onward [Robinson et al., 2007]. This is due to their unique 404

location near the inflection point in the Pamir arc, where they serve as a “ball bearing” to accommodate 405

oroclinal bending and extension between the northward advancing Pamir salient and the western 406

Kunlun range [Robinson et al., 2004]. Oroclinal bending during this time caused up to 30° clockwise 407

rotations along the eastern flank of the Pamir [Rumelhart et al., 1999], likely rotating the Karakax fault 408

and Jinsha suture into their current orientations. 409

Our proposed model is speculative and makes assumptions about several longstanding issues in 410

Pamir tectonics that are difficult to test. We assume that the northern Pamir terrane (or the northern 411

and central Pamir terranes) are equivalent to the Songpan Garze terrane [Burtman and Molnar, 1993; 412

Robinson, 2009; Schwab et al., 2004]. We also assume that the northernmost extension of the Karakax 413

fault runs east of the Mustagh and Kongur massifs where it links with the Ghez, Muji, and Markansu 414

fault systems, all of which mark the northern edge of the Songpan‐Garze/northern Pamir terrane 415

[Schwab et al., 2004]. Our model also assumes that the western portion of the Jinsha suture was 416

actively accommodating strike‐slip motion prior to being reactivated as a thrust system, and rotated 417

clockwise. The Ghoza‐Co fault system may represent a part of the suture still active as a strike‐slip 418

system today [Matte et al., 1996; Peltzer et al., 1989; Raterman et al., 2007]. 419

15

8. Conclusions 420

The Karakul graben is a prominent structural feature sitting atop the 75 km thick crust of the 421

northern Pamir plateau. Its proximity to the Kongur Shan extensional system caused speculation that 422

the Karakul Graben was a late Miocene‐Pliocene structural feature caused by radial thrusting, oroclinal 423

bending, or orogenic collapse. We present new apatite and zircon (U/Th)‐He ages from the Karakul 424

graben which document relatively slow cooling since ~220 Ma, punctuated by two episodes of rapid 425

exhumation between ~55‐40 Ma and ~24‐16 Ma. Based on the spatial pattern of cooling ages, we 426

argue that the second period of rapid cooling records tectonic exhumation due to the onset of extension 427

in the Karakul graben. We present bulk rock geochemistry data that supports the hypothesis of Schwab 428

et al. (2004) that the Karakul granites are associated with Mazar granites, and are therefore located 429

south of the Kunlun suture. Based on this reconstruction, we propose that extension in the Karakul 430

graben could have been facilitated by the onset of strike‐slip motion along the Karakax and Karakoram 431

faults at ~24 Ma. 432

433

434

435

436

437

438

439

440

441

442

443

444

445

446

16

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Yuan, C., M. Sun, M. Zhou, H. Zhou, W. Xiao, and J. Li (2002), Tectonic Evolution of the West Kunlun: 637 Geochronologic and Geochemical Constraints from Kudi Granitoids, Int. Geol. Review, 44, 653‐669. 638 639 Yuquan, Z., X. Yingwen, X. Ronghua, P. Vidal, and N. Arnaud (1996), Geochemistry of Granitoid rocks, in 640 Geological Evolution of the Karakorum and Kunlun Mountains, edited by P. Yusheng, Seismological Press, 641 Beijing, China. 642 643 644 645 646

647

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21

Figure Captions 677

678 Figure 1: Map showing the regional tectonics of the Pamir Plateau and Tarim Basin, synthesized from 679

Matte et al. (1986), Burtman and Molnar (1993), Searle (1996), Raterman et al. (2007), and 680 Robinson et al., (2009). Note that the Karakax fault is proposed to run along the eastern edge of 681 the Kongur massif, where it connects with the Ghez and Markansu fault systems. 682

683 Figure 2: Geologic map of the Karakul basin. Bedrock geology is taken from Romanko (1968). 684

Structures are from field observations, interpretations of satellite imagery, and from the 685 observations of Blisniuk et al. (1997). White boxes contain mean apatite (“a”) or zircon (“z”) 686 (U/Th)‐He ages. Grey boxes contain the data of Schwab et al. (2004), showing biotite 40Ar/40Ar 687 ages (“ArB”) and zircon U‐Pb ages (“UPbZ”). Hatched lines are normal faults, dashed where 688 inferred and solid where scarps are observed. 689

690 Figure 3: Age‐elevation profiles from each of the four transects shown in figure 2. All errors are 1σ, and 691

are ~7% for both apatite and zircon. Full dataset is available in online tables 1 and 2. 692 693 Figure 4: This figure compares the average REE (top) and trace element (bottom) distributions for the 694

Karakul granodiorites to other granitoids in western Tibet. The Karakul granodiorites are nearly 695 identical to the that of the Mazar granodiorite, and only slightly different from the Arkarz Shan 696 Intrusive Complex (ARIC) near Kudi [Yuan et al., 2002], and from the Muji granitoids [Yuquan et 697 al., 1996]. Filled circles in the upper panel show previously published data from the Karakul 698 granites [Schwab et al., 2004]. Data from granitoids elsewhere in Tibet are shown for 699 comparison and is taken from Yuquan et al. (1996). The Karakul, Muji, Mazar, and ARIC granites 700 are ~200‐230 Ma, whereas Rutog, Gheze and Tashkurgan are < 100 Ma, and Kudi is ~380 Ma. 701

702 Figure 5: Schematic diagram showing our proposed tectonic evolution for the Karakul Graben. We 703

propose that the Karakax originally connected to the Markansu fault, and that the Karakoram 704 fault originally terminated near the eastern end of the Rangkul fault. We speculate that 705 initiation of slip on the two major strike‐slip faults at ~24‐22 Ma, may have facilitated extension 706 in the Karakul Graben. Major structural features are labeled as: GCF= Ghoza Co fault, KG= 707 Karakul graben, JS= Jinsha suture, KF= Karakoram fault, KxF/KS= Karakax fault/ Kudi suture, MF= 708 Markansu fault, RF= Rangkul fault. 709

710 711

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Contour = 100 m0 4 8 km ´


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U/Th-He Age (My)

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ApatiteZircon

1σ errors

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Karakul

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Muji

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Table 1. U/Th‐He Cooling AgesSample Lat. Lon. Elev. Apatite Zircon

(wgs 84) (wgs 84) (m asl) (My) (My)

NE Transecttaj‐93 39.1010 73.6730 5579 17.2 24.6taj‐96 39.0934 73.6638 5094 19.1 22.2taj‐99 39.0846 73.6525 4663 17.7 22.4taj‐102 39.0776 73.6280 4212 15.3 21.0SW Transectp916‐1 38.8456 73.4178 4654 43.0 ‐‐‐p916‐2 38.8445 73.3942 5069 64.6 164.9p916‐3 38.8431 73.4198 4560 44.3 161.1p916‐4 38.8377 73.4348 4098 37.2 162.2SE Transectp917‐1 38.8247 73.5567 5465 39.8 172.7p917‐2 38.8298 73.5323 4900 31.5 162.2p917‐3 38.8373 73.5244 4497 17.1 154.9p917‐4 38.8451 73.5147 4112 15.4 139.4NW Transectp919‐1 39.1193 73.3018 3929 17.8 37.8p919‐2 39.1019 73.2496 4915 18.8 39.7p919‐3 39.1100 73.2812 4423 18.4 38.0p919 3 39.1100 73.2812 4423 18.4 38.0Lake Level p918‐1* 38.8402 73.3243 3947 69.9 96.5p918‐2 38.8429 73.2912 3950 32.9 ‐‐‐p919‐1a 39.1263 73.4203 3937 22.0 42.6StandardsDurango ‐‐‐ ‐‐‐ ‐‐‐ 32.5 ‐‐‐FCT ‐‐‐ ‐‐‐ ‐‐‐ ‐‐‐ 27.71σ errors are ~7% for both apatite and zircon

*poor reproducibility of apatite ages, 1σ error ~30%

Online Table 1: Apatite U/Th‐He ResultsSample U Th He FT Age FT corr age Sample U Th He FT Age FT corr age

(ppm) (ppm) (nmol/g) factor (My) (My) (ppm) (ppm) (nmol/g) factor (My) (My)

taj93a 20.9 29.4 2 0.82 13.2 16 P917‐4a 68.3 61.4 5.7 0.83 12.6 15.2taj93b 16.2 31.3 2.2 0.86 17 19.8 P917‐4b 50.5 24.2 4.2 0.84 14.0 16.7taj93c 29.4 29.9 2.5 0.81 12.7 15.7 P917‐4c 49.6 25.5 3.4 0.79 11.4 14.3taj93d 31.1 38.7 3.1 0.81 14 17.2 mean 15.4

mean 17.2 P918‐1a 18.9 7.6 4.6 0.84 40.8 48.3taj96a 36.6 5.2 3.4 0.80 16.3 20.2 P918‐1b 38.1 37.2 11.6 0.85 45.3 53.3taj96b 43.9 5.5 3.8 0.84 15.6 18.5 P918‐1c 62.2 58.0 19.4 0.81 46.9 57.9taj96c 74.8 5 6.2 0.80 14.9 18.6 P918‐1d 18.9 12.9 8.9 0.83 73.5 88.8

mean 19.1 P918‐1e 32.8 10.5 12.0 0.85 61.8 72.8taj99a 32.1 2.5 2.5 0.84 14.5 17.3 P918‐1f 42.4 117.3 31.5 0.83 82.1 98.3taj99b 34.4 0.8 2.7 0.83 14.5 17.5 mean 69.9taj99c 66.3 2.8 5.3 0.80 14.6 18.3 P918‐2a 8.4 12.8 1.7 0.77 26.6 34.4

14.5 17.7 P918‐2b 8.1 13.0 1.6 0.74 25.7 34.7taj102a 9.4 3.9 0.6 0.78 11.2 14.5 P918‐2c 8.4 13.2 1.7 0.82 26.4 32taj102b 18 11.4 1.4 0.73 12.3 16.8 P918‐2d 7.7 12 1.3 0.74 22.7 30.4taj102c 14.7 10.6 1 0.73 10.7 14.5 mean 32.9

11.4 15.3 P919‐1a(a) 23.6 16.8 2.6 0.78 17.1 21.8P916‐1a 39.2 29.4 9.9 0.85 39.2 45.9 P919‐1a(b) 21.5 6.7 2.2 0.79 16.9 21.3P916‐1b 76.3 61.1 17.0 0.71 34.4 48.2 P919‐1a(c) 19.9 19 2.3 0.76 17.4 22.9P916‐1c 133.8 97.5 23.1 0.66 27.0 40.6 mean 22.0P916‐1d 68.3 99.4 15.4 0.67 30.5 45.4 P919‐1a 75.7 33.2 6.6 0.79 14.5 18.5P916‐1e 39.5 45.0 6.1 0.62 22.0 35.1 P919‐1b 76.9 35.2 6.2 0.78 13.3 17.2

mean 43.0 P919‐1c 131.8 70.9 11.5 0.80 14.2 17.7P9162a 55.6 42.7 17.3 0.75 48.2 63.7 P919‐1d 16.3 22.6 2 0.85 16.6 19.4P9162b 61.1 61.7 23 0.80 55.5 69.3 14.0 17.8P9162c 47.1 17.2 14.4 0.84 51.1 60.9 P919‐2a 33.4 14 2.8 0.82 13.8 16.9

mean 64.6 P919‐2b 31.3 19.3 3.3 0.81 16.7 20.5P916‐3a 63.9 48.9 15.2 0.79 36.9 46.9 P919‐2c 45.8 26.6 4.5 0.80 16.0 19.8P916‐3b 56.4 39.7 14.3 0.84 40.8 48.4 P919‐2d 32.3 14.4 3.0 0.85 15.4 18.1P916‐3c 34.5 15.0 6.6 0.85 31.8 37.5 P919‐2e 40.2 35.2 3.6 0.80 13.8 17.2

mean 44.3 P919‐2f 11.3 16.2 1.4 0.82 16.7 20.4P916‐4a 56.4 28.6 10.2 0.81 29.5 36.4 mean 18.8P916‐4b 63.9 64.2 13.2 0.82 30.7 37.3 P919‐3a 30.0 10.2 4.4 0.81 24.8 30.5P916‐4c 52.9 55.7 11.3 0.83 31.4 37.9 P919‐3b 18.7 13.0 1.7 0.83 15.1 18.2

mean 37.2 P919‐3c 18.7 8.1 1.5 0.83 13.9 16.8P917‐1a 38.9 59.8 8.5 0.82 29.5 35.9 P919‐3d 11.1 3.1 1.1 0.79 16.0 20.3P917‐1b 78.1 75.7 17.8 0.81 34.1 42.2 mean 18.4P917‐1c 62.0 66.2 14.5 0.81 34.3 42.1 Durango Apatite StandardsP917‐1d 80.0 76.8 16.6 0.79 31.0 39.1 dur1 9.2 195.9 8.2 0.82 27.47 33.7

32.2 39.8 dur2 13.3 253.4 10.2 0.85 25.75 30.5P917‐2a 68.4 70.6 12.7 0.82 27.5 33.6 dur3 12.9 264.4 11.2 0.87 27.494 31.7P917‐2b 87.8 83.0 15.1 0.85 25.9 30.4 dur4 8.3 168.8 7.6 0.87 29.34 33.8P917‐2c 77.0 62.0 11.5 0.80 23.0 28.8 dur5 11.3 202.7 10.1 0.96 31.5 32.8P917‐2d 56.0 73.6 11.3 0.85 28.2 33.3 dur6 9.4 195.9 9.7 0.96 32.1 33.4

mean 31.5 dur7 11.7 205.6 10.1 0.96 30.8 32.0P917‐3a 72.1 45.6 6.9 0.86 15.4 17.9 mean 32.5P917‐3b 32.2 32.2 2.6 0.78 12.1 15.6P917‐3c 79.6 73.6 7.1 0.79 13.4 17.0P917‐3d 74.8 65.5 7.6 0.85 15.4 18.1

mean 17.1

Online Table 2: Zircon U/Th‐He ResultsS l U Th H FT A FT S l U Th H FT A FTSample U Th He FT Age FT corr age Sample U Th He FT Age FT corr age

(ppm) (ppm) (nmol/g) factor (My) (My) (ppm) (ppm) (nmol/g) factor (My) (My)

taj93‐a 2506 1641 331.3 0.83 21.2 25.6 P917‐3a 675 160 517.3 0.82 133.3 162.3taj93‐b 1512 289 168.9 0.84 19.7 23.5 P917‐3b 771 131 553.6 0.82 126.7 153.8taj93‐c 979 553 142.6 0.89 22.1 24.8 P917‐3c 605 83 419.9 0.83 123.6 148.7

mean 24.6 mean 154.9taj96‐a 175 64 16.6 0.74 16.2 21.8 P917‐4a 678 139 427.3 0.81 110.5 136.4taj96‐b 274 56 26.6 0.76 16.4 21.5 P917‐4b 317 89 217.0 0.81 117.8 145.1taj96‐c 215 60 21.8 0.76 17.8 23.4 P917‐4c 940 95 574.6 0.80 109.8 136.7

mean 22.2 mean 139.4taj99‐a 700 238 68.2 0.79 16.8 21.2 p918‐1a 1067 119 459.5 0.78 77.5 99.0taj99‐b 1036 164 125.3 0.88 21.6 24.5 p918‐1b 990 89 386.5 0.75 70.6 94.1taj99‐b 1036 164 125.3 0.88 21.6 24.5 p918‐1b 990 89 386.5 0.75 70.6 94.1taj99‐c 1291 117 109.6 0.74 15.4 20.7 mean 96.5taj99‐d 1318 123 127.4 0.76 17.5 23.2 P919‐1a(a) 1155 229 218.6 0.78 33.5 42.7

mean 22.4 P919‐1a(b) 590 118 116.0 0.77 34.7 44.9taj102‐a 945 205 80.9 0.79 15.1 19.1 P919‐1a(c) 862 144 169.6 0.74 35.1 47.2taj102‐b 263 101 26.9 0.82 17.3 21.2 P919‐1a(d) 646 112 102.3 0.79 28.2 35.7j102 254 554 37 8 0 80 18 2 22 8 42 6taj102‐c 254 554 37.8 0.80 18.2 22.8 mean 42.6

mean 21.0 P919‐1a 479 103 86.5 0.84 32.1 38.1P916‐2a 1406 177 1092.9 0.84 138.5 165.2 P919‐1b 643 140 128.9 0.87 35.3 40.6P916‐2b 1172 99 823.1 0.80 126.5 157.1 P919‐1c 646 148 114.8 0.88 30.5 34.8P916‐2c 190 55 148.6 0.78 134.5 172.5 32.6 37.8

mean 164.9 P919‐2a 534 108 99.8 0.79 33.0 41.7mean 164.9 P919 2a 534 108 99.8 0.79 33.0 41.7P916‐3a 593 52 479.7 0.90 145.4 161.1 P919‐2b 1491 441 303.9 0.82 35.3 43.1P916‐3b 1240 108 730.8 0.73 106.3 145.6 P919‐2c 2174 424 312.1 0.74 25.4 34.2

mean 161.1 mean 39.7P916‐4a 453 97 352.2 0.84 135.7 161.1 P919‐3a 996 160 163.4 0.86 29.3 34.2P916‐4b 314 130 256.4 0.83 136.5 164.7 P919‐3b 875 185 154.3 0.83 31.0 37.4P916 4c 300 35 280 4 0 81 166 2 205 2 P919 3c 1055 205 190 9 0 75 32 1 42 5P916‐4c 300 35 280.4 0.81 166.2 205.2 P919‐3c 1055 205 190.9 0.75 32.1 42.5P916‐4d 474 87 344.2 0.79 127.8 160.8 mean 38.0

mean 162.2 Fish Canyon Tuff StandardsP917‐1a 248 65 241.5 0.80 134.1 167.0 FCT‐1 367 147 49 0.78 22.5 28.8P917‐1b 417 77 334.6 0.79 141.0 178.8 FCT‐2 195 120 26 0.81 21.7 26.8P917‐1c 630 78 467.8 0.77 132.5 172.3 FCT‐3 211 108 28 0.80 22.0 27.5

mean 172.7 mean 27.7P917‐2a 486 54 349.3 0.82 128.6 156.5P917‐2b 268 106 214.2 0.80 134.0 167.9

mean 162.2

Online Table 3: Mineral compositions determined by electron microprobe analysis (Oxide Wt. %)Taj93 n Na2O MgO TiO2 Cr2O3 K2O CaO SiO2 Al2O3 FeO MnO Total

K‐feldspar 6 0.6 0.0 0.0 0.0 15.9 0.0 65.4 18.4 0.1 0.0 100.51σ 0.14 0.01 0.01 0.01 0.25 0.01 0.32 0.07 0.05 0.01 0.43

Plagioclase 6 6.5 0.0 0.0 0.0 0.2 8.9 57.3 26.8 0.0 0.0 99.71σ 0.08 0.01 0.00 0.01 0.03 0.07 0.44 0.18 0.03 0.02 0.53

Hornblende 30 0.9 15.0 0.3 0.1 0.5 12.0 49.5 7.3 10.9 0.3 96.81σ 0.04 0.22 0.04 0.02 0.03 0.08 0.40 0.34 0.20 0.03 0.18

Epidote 2 0.3 0.0 0.0 0.0 0.0 23.1 40.3 30.3 2.3 0.1 96.5Muscovite 3 0.2 1.9 0.1 0.2 10.9 0.0 46.7 31.2 1.8 0.0 93.2Also contains quartz >>> ilmenite > titanite > apatite > zircon.

Taj96 n Na2O MgO TiO2 Cr2O3 K2O CaO SiO2 Al2O3 FeO MnO Total

Plagioclase 4 7.0 0.0 0.0 0.0 0.1 7.7 58.9 25.8 0.0 0.0 99.61σ 0.33 0.00 0.00 0.02 0.02 0.71 1.18 0.62 0.04 0.01 0.44

Biotite 9 0.1 3.7 3.0 0.0 9.3 0.0 33.8 17.2 27.3 0.3 94.71σ 0.03 0.23 0.29 0.01 0.40 0.06 0.39 0.64 0.60 0.04 0.86

Garnet 6 0.0 1.2 0.1 0.0 0.0 4.7 37.3 19.9 33.8 3.3 100.31σ 0.02 0.16 0.02 0.00 0.00 0.26 0.80 0.51 0.23 0.29 1.47

Titanite 1 0.0 0.0 14.2 0.0 0.0 25.4 34.9 17.4 6.8 0.1 98.7Also contains quartz >>> muscovite >apatite > zircon.


Plagioclase 4 7.3 0.0 0.0 0.0 0.1 7.3 59.6 25.3 0.1 0.0 99.71σ 0.12 0.01 0.00 0.00 0.01 0.10 0.25 0.14 0.05 0.01 0.42

Tourmaline 4 0.0 6.4 0.1 0.0 0.0 0.1 23.9 20.5 34.7 0.6 86.31σ 0.03 0.18 0.06 0.01 0.01 0.03 0.34 0.23 0.15 0.04 0.73

Muscovite 5 0.2 1.3 0.3 0.0 11.0 0.0 46.8 30.4 3.4 0.0 93.41σ 0.08 0.26 0.33 0.02 0.10 0.02 0.31 1.18 0.70 0.01 0.01

Biotite 2 0.1 4.5 1.8 0.0 8.4 0.3 34.2 18.1 25.6 0.4 93.4Titanite 1 0.0 0.0 33.8 0.0 0.1 29.5 32.2 3.7 0.4 0.1 99.8Also contains quartz >>> apatite > zircon.


K‐feldspar 6 0.6 0.0 0.0 0.0 16.0 0.0 64.1 18.3 0.0 0.0 99.01σ 0.18 0.01 0.02 0.00 0.38 0.03 0.32 0.34 0.02 0.01 0.43

Plagioclase 4 8.0 0.0 0.0 0.0 0.2 6.7 59.5 24.6 0.0 0.0 99.01σ 0.20 0.01 0.00 0.01 0.06 0.36 0.76 0.30 0.01 0.01 0.64

Biotite 7 0.1 4.7 2.4 0.0 9.2 0.1 34.4 17.1 25.9 0.4 94.21σ 0.04 0.61 0.17 0.01 0.42 0.06 1.28 0.58 0.58 0.04 0.93

Garnet 6 0.0 0.8 0.0 0.0 0.0 2.9 36.7 19.8 28.6 10.4 99.31σ 0.00 0.05 0.10 0.00 0.00 0.72 0.48 0.23 0.68 0.45 0.37

Epidote 2 0.0 0.0 0.2 0.0 0.0 23.9 37.3 24.4 9.0 0.2 94.9Also contains quartz >>> muscovite>apatite >zircon > titanite.

Online Table 4: Bulk Rock GeochemistryP916 1 P916 P917 P919P916‐1 P916 P917 P919

SiO2 40.5 64.3 67.1 63.9TiO2 1.3 0.5 0.4 0.6Al2O3 12.4 16.2 15.6 16.2Fe2O3 10.1 4.5 3.9 5.0MnO 0.2 0.1 0.1 0.1MgO 12.1 2.2 1.2 2.5gCaO 10.2 4.3 3.1 4.4Na2O 3.9 2.5 3.3 2.4K2O 1.9 3.5 3.7 3.5P2O5 2.4 0.2 0.1 0.2LOI 4.3 1.6 1.3 1.3Total: 99.31 99.85 99.89 99.86

Sr 2946.0 414.0 186.0 385.0Ni 347.0 bd bd bdCu 66.0 9.0 11.0 13.0Zn 77.0 58.0 54.0 56.0Rb 35.0 152.0 142.0 140.0Zr 86.0 125.0 141.0 94.0B 703 2 664 0 561 2 659 9Ba 703.2 664.0 561.2 659.9La 112.6 37.1 23.5 29.0Ce 172.1 96.7 58.2 75.2Pr 16.0 8.7 5.8 7.1Nd 50.4 29.0 20.2 24.6Sm 7.1 4.9 4.0 4.4Eu 1.8 1.3 1.0 1.2Gd 7.0 4.8 4.1 4.3Tb 0.8 0.7 0.6 0.6Y 13.1 14.5 20.6 14.9Dy 2.9 2.5 3.2 2.5Ho 0.6 0.5 0.6 0.5Er 1.5 1.3 1.9 1.4Yb 1.4 1.5 2.0 1.4Yb 1.4 1.5 2.0 1.4Lu 0.2 0.3 0.3 0.2V 66.2 54.2 25.4 67.5Cr 200.6 27.0 7.9 31.4Nb 84.5 17.2 19.8 15.8Hf 2.1 3.1 3.4 2.5Ta 2.2 1.6 1.8 1.2Pb 7 3 77 4 44 2 51 7Pb 7.3 77.4 44.2 51.7Th 14.9 16.9 10.3 15.5U 2.5 2.7 2.4 2.4

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