RESEARCH Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir Barbara Carrapa1,*, Fariq Shazanee Mustapha1, Michael Cosca2, George Gehrels1, Lindsay M. Schoenbohm3, Edward R. Sobel4, Peter G. DeCelles1, Joellen Russell1, and Paul Goodman1 1DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721, USA 2U.S. GEOLOGICAL SURVEY, DENVER FEDERAL CENTER, DENVER, COLORADO 80225, USA 3DEPARTMENT OF CHEMICAL AND PHYSICAL SCIENCES, UNIVERSITY OF TORONTO–MISSISSAUGA, MISSISSAUGA, ONTARIO L5L 1C6, CANADA 4INSTITUTE OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF POTSDAM, POTSDAM-GOLM 14476, GERMANY ABSTRACT The Pamir is the western continuation of Tibet and the site of some of the highest mountains on Earth, yet comparatively little is known about its crustal and tectonic evolution and erosional history. Both Tibet and the Pamir are characterized by similar terranes and sutures that can be correlated along strike, although the details of such correlations remain controversial. The erosional history of the Pamir with respect to Tibet is significantly different as well: Most of Tibet has been characterized by internal drainage and low erosion rates since the early Cenozoic; in contrast, the Pamir is externally drained and topographically more rugged, and it has a strongly asymmetric drainage pattern. Here, we report 700 new U-Pb and Lu-Hf isotope determinations and >300 40Ar/ 39Ar ages from detrital minerals derived from rivers in China draining the northeastern Pamir and >1000 apatite fission-track (AFT) ages from 12 rivers in Tajikistan and China draining the northeastern, central, and southern Pamir. U-Pb ages from rivers draining the northeastern Pamir are Mesozoic to Proterozoic and show affinity with the Songpan-Ganzi terrane of northern Tibet, whereas rivers draining the central and southern Pamir are mainly Mesozoic and show some affinity with the Qiangtang terrane of central Tibet. Theε Hf values are juvenile, between 15 and −5, for the northeastern Pamir and juvenile to moderately evolved, between 10 and −40, for the central and southern Pamir. Detrital mica 40Ar/39Ar ages for the northeastern Pamir (eastern drainages) are generally older than ages from the central and southern Pamir (western drainages), indicating younger or lower-magnitude exhumation of the northeastern Pamir compared to the central and southern Pamir. AFT data show strong Miocene–Pliocene signals at the orogen scale, indicating rapid erosion at the regional scale. Despite localized exhumation of the Mustagh-Ata and Kongur-Shan domes, average erosion rates for the northeastern Pamir are up to one order of magnitude lower than erosion rates recorded by the central and southern Pamir. Deeper exhumation of the central and southern Pamir is associated with tectonic exhumation of central Pamir domes. Deeper exhumation coincides with western and asymmetric drainages and with higher precipitation today, suggesting an orographic effect on exhumation. A younging-southward trend of cooling ages may reflect tectonic processes. Overall, cooling ages derived from the Pamir are younger than ages recorded in Tibet, indicating younger and higher magnitudes of erosion in the Pamir. LITHOSPHERE; v. 6; no. 6; p. 443–455; GSA Data Repository Item 2014332 | Published online 27 August 2014 doi: 10.1130/L360.1 INTRODUCTION of the Pamir with respect to Tibet but differ in Rohrmann et al., 2012). Cenozoic exhuma- the magnitude of offset along the Karakorum tion is localized around Miocene rifts (Kapp The Pamir Mountains form the western fault as well in the width of individual geologic and Guynn, 2004) and the southeastern exter- pro longation of the Tibetan-Himalayan col- terranes across the orogenic system and in the nally drained margin of the plateau (Clark et lisional orogenic system (Fig. 1), which is the degree of correlation (Fig. 2). In particular, al., 2005) and on the frontal (southern) flank locus of Earth’s highest mountains and largest early work by Tapponnier et al. (1981) and the of the Himalaya (Thiede and Ehlers, 2013). continental plateau. Although both Tibet and more recent correlation of Schwab et al. (2004) Because of the aridity, glaciers are for the most the Pamir are characterized by similar rocks call for a large magnitude of slip (~250 km), part restricted to the crests of mountain ranges and tectonostratigraphic architecture (Şengör, whereas others (Searle, 1996; Robinson et al., north of the Himalaya (Owen, 2009), and 1984; Dewey et al., 1988; Burtman and Mol- 2004; Robinson, 2009) suggest a much smaller glacial erosion has not significantly affected nar, 1993; Schwab et al., 2004; Robinson et al., offset (~150 km). landscape morphology. The Pamir, in contrast, 2007, 2012; Robinson, 2009), current debate Tibet and the Pamir are strikingly different is mostly externally drained, has high topo- exists about the exact correlation of terranes in terms of morphology and exhumation his- graphic relief (>2–3 km), and contains widely and sutures along strike (Schwab et al., 2004; tory. Tibet is largely characterized by internal exposed high-grade metamorphic domes that Robinson et al., 2004, 2012; Robinson, 2009). drainage, high elevation, relatively low inter- have been exhumed since early Miocene time All models suggest northward displacement nal relief (Fielding et al., 1994), and limited (Fig. 1; Schwab et al., 2004; Schmidt et al., erosion since the early Cenozoic (Rowley and 2011; Lukens et al., 2012). A striking feature *[email protected] Currie, 2006; DeCelles et al., 2007a, 2007b; of the Pamir is the asymmetry in morphology LITHOSPHERE© 2014 Geological | Volume Society 6 of| AmericaNumber 6| |For www.gsapubs.org permission to copy, contact [email protected] 443 CARRAPA ET AL. 444 68°E 70°E 72°E 74°E 76°E 78°E A EURASIA 40°N an) Alai 0 50 100 150 km n Sh Valley 40°N ie e (T Kashgar (Kashi) Alai Rang Tajik depression Tarim Basin Pamir s-Alai Range Tran 1071-4 Muji N 1071-5 Oytag Tibet Northern Pamir 1071-1 30°N 1071-2 T Central Pamir arim TJK-04 Ba INDIA TJK-08 si Murghab Tashkorgan n TJK-07 A 1071-3 20°N ks 38°N t u 1071-7 l TJK-06 - u Fa M 1071-6 a u u F Southern Pamir lt r z TJK-05 gh a a 70°E 80°E 90°E 100°E v Khorog b r a D K a Tajik depression r A a lt k y o n-T r agh Hindu Kush a Fa m ult F B a u l t 36°N www.gsapubs.org Sed-Metased Rocks Igneous Rocks River sand samples Quaternary Cenozoic Western Pamir Thrust Fault TJK4 = Murghab River Neogene-Paleogene Triassic-Jurassic Normal fault TJK5 = Gunt River | TJK6 = Bartang River Triassic-Jurassic Cretaceous Volume 6 Volume Strike Slip Fault TJK7 = Yazgulem River Cretaceous Paleozoic Town Locations TJK8 = Vanj River Northeastern Pamir Paleozoic/ Precambrian Sample Locations | 1071- 3, 6, 7 = Tashkorgan River tributaries Number 6 undierentiated 1071- 1, 2 = Gez River 1071- 4 = Kalate River Figure 1. (A) Inset map showing political borders. (B) Simplified geologic map of the Pamir, showing lithologic units and main sutures, compiled after Bershaw et al. (2012), Lukens et al. | (2012), Robinson et al. (2007), and Teraoka and Okumura (2007). LITHOSPHERE Multisystem dating of modern river detritus from the Pamir | RESEARCH 70°E 80°E90°E100°E and climate from west to east (Fig. 3). The Tien Shan 40°N A west side of the range receives up to 60 cm/yr Altyn Tagh Fault 40°N in rainfall, dominantly delivered by the midlat- A MPT Tajik B Tarim Basin itude westerlies during the spring (Aizen et al., RPS TS depression 2001). Rivers are deeply incised, and the drain- Pamir Qaidam C SS age divide is displaced far to the east within D AKMS the range. The western Pamir hosts some of Songpan-Ganzi the largest glaciers outside of the polar regions K JS a Qiangtang ra k and the Himalaya and Karakoram (Fuchs et o r Tibetan Plateau a 30°N m al., 2013), including the Fedchenko glacier in F a BNS u lt Lhasa 30°N Tajikistan. Precipitation drops off dramatically eastward across the Pamir to less than 10 cm/yr Him IYS M alayas by the midpoint of the range. Glaciers and per- ain Fro ntal manent snow cover correspondingly decline India Thr ust 0 200km (Fuchs et al., 2013). Eastern drainages are much less dissected and are dominated by the 70°E 80°E90°E 100°E structural basin in the hanging wall of the Kon- gur extensional system (Robinson et al., 2004). 70°E 80°E 90°E 100°E A wealth of thermochronological, geochro- Tien Shan 40°N B nological, and structural data exists from Tibet Altyn Tagh Fault 40°N (e.g., Copeland et al., 1995; Ratschbacher et al., A MPT Tajik Tarim Basin B RPS TS 1996; Murphy et al., 1997; Kapp et al., 2007; depression Pamir Qaidam Jolivet et al., 2001; Kirby et al., 2002; Rohrmann C SS D AKMS et al., 2012), whereas only sparse data exist for the Pamir (e.g., Amidon and Hynek, 2010; Rob- Songpan-Ganzi K JS a Qiangtang inson et al., 2012; Sobel et al., 2013; Stübner et ra k o r Tibetan Plateau al., 2013a, 2013b; Thiede et al., 2013), leaving a 30°N m F a BNS u the exhumation and tectonic history of this part lt Lhasa 30°N of the orogenic system largely unresolved. Here, Him IYS we apply geochronology, thermochronology, M alayas ain and Hf isotope geochemistry to modern river Fro ntal India Thr sand grains in order to determine the timing of ust 0 200km crustal evolution and the timing and pattern of 70°E 80°E 90°E 100°E exhumation of the Pamir.