ÔØ Å ÒÙ× Ö ÔØ Exhumation and incision history of the Lahul Himalaya, northern India, based on (U-Th)/He thermochronometry and terrestrial cosmogenic nuclide methods ACCEPTED MANUSCRIPT

Exhumation and incision history of the Lahul Himalaya, northern India, based on (U-Th)/He thermochronometry and terrestrial cosmogenic nuclide methods, Geomorphology (2009), doi: 10.1016/j.geomorph.2008.12.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Low-temperature apatite (U-Th)/He (AHe) thermochronology on vertical transects of 22 leucogranite stocks and 10 Be terrestrial cosmogenic nuclide (TNC) surface exposure dating on 23 strath terraces in the Lahul Himalaya provide a first approximation of long-term (10 4 -10 6 years) 24 exhumation rates for the High Himalayan Crystalline Sequence (HHCS) for northern India. The 25 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT AHe ages show that exhumation of the HHCS in Lahul from shallow crustal levels to the surface 26 was ∼ 1-2 mm/a and occurred during the past ∼ 2.5 Ma. Bedrock exhumation in Lahul fits into a 27 regional pattern in the HHCS of low-temperature thermochronometers yielding Plio-Pleistocene 28 ages. Surface exposure ages of strath terraces along the Chandra River range from ∼ 3.5 to 0.2 29 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT ka. Two sites along the Chandra River show a correlation between TCN age and height above 30 the river level yielding maximum incision rates of 12 and 5.5 mm/a. Comparison of our AHe 31 and surface exposure ages from Lahul with thermochronometry data from the fastest uplifting 32 region at the western end of the Himalaya, the Nanga Parbat syntaxis, illustrates that there are 33 contrasting regions in the High Himalaya where longer term (10 5 -10 7 years) erosion and 34 exhumation of bedrock substantially differ even though Holocene rates of fluvial incision are 35 comparable. These data imply that the orogen's indenting corners are regions where focused 36 denudation has been stable since the mid-Pliocene. However, away from these localized areas 37 where there is a potent coupling of tectonic and surface processes that produce rapid uplift and 38 denudation, Plio-Pleistocene erosion and exhumation can be characterized by disequilibrium, 39 where longer term rates are relatively slower and shorter term fluvial erosion is highly variable 40 over time and distance. The surface exposure age data reflect differential incision along the 41 length of the Chandra River over millennial time frames, illustrate the variances that are possible 42 in Himalayan river incision, and highlight the complexity of Himalayan environments. 43 45 Keywords: Himalaya; strath terraces; terrestrial cosmogenic nuclides; AHe thermochronology; 46 exhumation; fluvial incision; Lahul 47 48 49 50 1. Introduction 51 52 Processes at convergent plate boundaries that build topography are widely understood to 53 be episodic on timescales of 10 6 -10 7 years (for example, Lamb et al., 1997; Lister et al., 2001; 54 Quarles van Ufford and Cloos, 2004). Transient landscapes, too, can persist on time scales of 55 10 6 years (Kirby et al., 2002; A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT transient or can achieve steady-state conditions remain important questions in geomorphology. 58 Key processes in addressing these issues are exhumation and erosion. The rates of these 59 processes constrain the interplay and relative roles of tectonic vs. surficial geologic processes in 60 mountain belts. 61 62 The Himalayan orogen is an archetype natural laboratory for the study of exhumation and 63 erosion because it is tectonically active and characterized by extreme relief (relative relief can 64 exceed 3000 m), large-scale mass wasting (large avalanches, debris flows, and rock falls), and 65 glacial landforms (over steepened valleys, moraines, and glacial dam bursts). Exhumation rates 66 of the northern Indian Himalaya have not been well defined in spite of their significance for 67 surficial and tectonic dynamics. To further understand the timing and rates of exhumation and 68 erosion in the Lahul region of the Greater Himalaya, we have obtained quantitative data using 69 (U-Th)/He apatite (AHe) thermochronology and terrestrial cosmogenic nuclide (TCN) methods. 70 71 Lahul is located approximately midway between the Indo-Gangetic Plain and Tibet 77 Several general aspects of the exhumation history of the Lahul Himalaya are well 78 characterized. These are derived from studies of regional deformation and faulting (Steck et al., 79 1993; studied, including catastrophic flooding 92 To build on these studies, we employed low-temperature AHe thermochronology on 93 vertical transects of leucogranite stocks and 10 Be terrestrial cosmogenic nuclide (TCN) surface 94 exposure dating (SED) on strath terraces exposed along the Chandra River and one of its 95 tributaries. Our primary goals in using AHe thermochronology in Lahul were first, to determine 96 whether long-term (10 6 years) exhumation rates could be established, and second, to gather data 97 bearing on whether the topographic and thermal structure of Lahul have reached steady-state. 98 Changes in erosion rate and the rate at which topography develops can significantly affect the 99 migration and geometry of isotherms and can disturb cooling ages at the surface (Braun et al., 100 2006, p.105-176). TNC methods can quantify surface processes at millennial timescales back to 101 20-30 ka, and our goal of dating strath terraces was to determine recent river incision rates. Any 102 spatial and temporal variation in surface exposure ages of strath terraces along the Chandra will 103 provide a gauge of the heterogeneity of fluvial bedrock incision in this active Himalayan 104 environment. 105 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT Our data can be used to test whether the Lahul Himalaya has undergone rapid 106 exhumation. i.e. 3-7 mm/a, as proposed for elsewhere in the orogen and to determine whether 107 local river incision rates are as high as other regions of the Himalaya, of the order of 1 to 20 108 mm/a, where more is known about uplift and erosion histories Background 114 115 Two main NW-SE-trending mountain ranges traverse Lahul, the Pir Panjal to the south 116 and the Greater Himalaya to the north ( Cambrian-Ordovician age Crustal thickening in Lahul has been viewed as occurring during emplacement of southwest-129 verging nappes during the late Eocene to early Oligocene, and again during the late Oligocene 130 and early Miocene coincident with movement along the northwest-dipping Main Central thrust 131 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT (MCT; semi-arid environment of Lahul is considerably less than that in the Lesser Himalaya due to 157 orographic effects. 158 159 The Chandra and Bhaga Rivers are the principal drainages of this region and they have 160 many smaller tributary streams originating from the surrounding steep mountainsides Within the study region, the Chandra River's stream order is a 3 on the Strahler scale as derived 162 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT Valley show insignificant bedload transport between high magnitude monsoon storm events and 181 the beds are clearly armored during this time. However, the data gathered at these stations are 182 limited, as only one month was recorded. 183 184 Lahul is similar to other regions of the Himalaya in that its river systems yield very high 185 sediment loads (Owen et al., 1995). Sediment transfer is episodic and dictated by seasonal 186 cycles, the magnitude of monsoon storm events, and the dynamics of highly active slope 187 processes (Owen et al., 1995). The Chandra oscillates from low width, single channel conditions 188 to wide, multi-channel braided sandur (glacial outwash plain) reaches along its length. Northern Pakistan since 720 ka Methods 222 Field mapping 223 224 We use the mapping of A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT throughout our research area. Surveys of strath terraces were undertaken using a hand-held laser 228 distance finder, an inclinometer, and a 30 m measuring tape. 229 230 DEM analysis 231 232 The best publicly available topographic data for the region consist of on-demand ASTER 233 (Advanced Spaceborne Thermal Emission and Reflectance) satellite digital elevation models 234 (DEMs) with 30 m cell size. Experience shows that the smallest landforms that can be identified 235 and mapped on a DEM have characteristic lengths approximately an order of magnitude larger 236 than the DEM cell size. Thus, a 30 m ASTER DEM is sufficient to identify landforms with 237 lengths on the order of 300 m or more. This is too coarse for geomorphic mapping of all but very 238 large landforms, but does provide a useful topographic framework for our work. 239 240 We obtained an ASTER DEM tile covering most of the project area and used it to extract 241 topographic profiles at approximately equal intervals and nearly perpendicular to the Chandra 242 River valley. Each profile was about 10 km long. Both the DEM (shown as a shaded relief 243 image) and the profiles are shown in 256 In the Hamptah Valley (on the south side of the Chandra valley; Valley (north side of the Chandra; 262 On the Rohtang Pass Sampling for 10Be TCN SED 269 270 Fifteen samples for 10 Be TCN surface exposure dating were collected from four strath 271 terraces along the Chandra River between Chattru and Koksar to define downstream variations in 272 incision, and one strath terrace along a tributary stream near its intersection with the Chandra 273 siltstone, or vein quartz) were collected from the different horizontal strath terrace surfaces (one 275 per level). Details of the TCN samples we collected are given in Specific locations to collect samples were selected on nearly horizontal surfaces of larger treads 287 and terraces exhibiting polish, potholes, and small sinuous channels were preferentially sampled 288 (Figs. 7C, 7E), as these treads have experienced less subsequent erosion since abandonment than 289 flat, featureless treads. Terraces that showed weathering features, including rough surfaces, deep 290 weathering pitting and exfoliation, were not sampled, nor were strath terraces that had any 291 sediment cover. 292 293 The Chandra River has low-width single channel reaches where strath terraces are present 294 Chandra are debris-covered slopes. It is possible that all of these terraces were formed in paired 297 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT successions, and since abandonment, one side of the valley has experienced mass wasting events 298 that have obscured the adjacent set of terraces. Though these debris covers have not been dated, 299 their presence indicates that they are long-lived relative to the geomorphic timescale (10 2 -10 6 300 years) and not easily cleared, and implies that there should be evidence of debris cover on strath 301 terrace surfaces if they were once covered. An effective mechanism to clear a bedrock terrace 302 surface perched 10 m or higher above river level would be a catastrophic flooding event. Flood 303 deposits along the Chandra AHe thermochronology 306 307 The low-temperature AHe thermochronometer allows recent exhumation of rocks to be 308 quantified in terms of cooling histories, typically on million-year timescales. (U-Th)/He dating 309 is based on the radiogenic production and thermally-controlled diffusion of 4 He within host 310 minerals. Studies of 4 He diffusion in apatite show that helium begins to be quantitatively 311 retained at ∼ 80°C Results 368 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 369 Digital Terrain Modeling 370 371 Although we did not find the DEM or derivative products (e.g., slope angle, aspect, 372 roughness, or curvature maps) useful for detailed geomorphic mapping in the project area, it did 373 yield topographic profiles showing four terrace levels that can be correlated from profile to 374 profile Profiles D-D', E-E', and F-F' contain several distinct terraces that are close, but distinctly 378 different than, the elevations projected from profile A-A'. They are shown with a query in 380 381 Because most of the queried terraces are slightly below the terrace elevations projected 382 from profile A-A', it is unlikely that they reflect upstream elevation increases along the stream 383 gradient. They may, however, indicate tectonic activity such as greater uplift or warping along 384 more deeply incised portions of the valley, or terrace levels not apparent in the other profiles. At 385 present, we are unable to distinguish between those two possibilities. Topographic anomalies 386 along the north side of the Chandra valley in the vicinity of profiles C-C', D-D', and F-F', which 387 are visible on both the shaded relief image and the profiles, appear to represent a complex of 388 large scale landslides and alluvial fans. 389 390 AHe data 391 392 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT AHe ages from five leucogranite samples in the Hamptah Valley are Plio-Pleistocene 393 Given that we did not measure 147 Sm on the initial runs of these reconnaissance samples, the 414 individual ages for low U samples could thus be too old if 147 Sm concentrations were higher than 415 we consider analyses based on less than ~ 0.4 femtomoles to be unreliable A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT Average age determinations were generally based on only two replicates per sample, and include 424 individual analyses in which the U content was < 3 ppm. Accordingly, we consider all five 425 average ages from this area to be less accurate than the standard deviation of individual 426 replicates. Nevertheless, these data do place a first-order constraint on the cooling history of this 427 of the Chandra that we sampled, surface exposure ages of strath treads range between 2.6±0.3 437 and 0.9±0.1 ka, and similarly calculated rates of incision range between 12.3±1.9 and 0.6±0.6 438 mm/a. Our weighted mean rate of incision including all of our data is 2.2±1.2 mm/a. If we 439 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT exclude the lowest four treads that were 1.5 m or less above the water level when they were 440 collected, the weighted mean rate of incision is 3.5±1.3 mm/a. 441 442 For two of the four strath terraces we sampled along the Chandra River there is a 443 correlation between surface height and age 448 For locations KL and PT, there is an intermediate-level strath that has a young surface 449 exposure age 453 As can be seen in Discussion 462 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 463 AHe thermochronology 464 465 Our AHe ages record recent exhumation of the HHCS in Lahul and fit into the regional 466 pattern of low-temperature thermochronometers in the HHCS yielding Plio-Pleistocene ages 467 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT hinterland structures or out-of-sequence thrust systems (Macfarlane et al., 1992; Harrison et al., 509 1997; Catlos et al., 2001 Even the most conservative interpretation of our AHe data -using sample RH2-1 and its error 515 AHe closure temperature have a strong effect on the slope of AERs, with changes in slope 521 affected by topographic wavelength (the horizontal distance between ridge crests), exhumation 522 rate, the geotherm, and the timescale of the change of surface relief (Braun, 2002). Within the 523 limits of our study area, the largest separation of ridge crests is across the Chandra Valley ACCEPTED MANUSCRIPT The youngest mean AHe age we obtained is 1.37±0.23 Ma from the Rohtang Pass. Such 531 young ages suggest that bedrock in our field area was exhumed through cooler, higher crustal 532 levels at the same time or before rocks elsewhere in the HHCS cooled through the higher AFT 533 and ZFT closure temperatures of ∼90-120°C and ~ 180-240°C, respectively (Gleadow and 534 Duddy, 1981; Surface exposure ages of strath terraces 543 544 As summarized by ACCEPTED MANUSCRIPT erosion rates of meandering channels In the field, the character and preservation of higher-level strath terrace surfaces suggests 579 that they have not undergone any significant weathering or erosion since their fluvial incision 580 and abandonment but it is possible that intermediate-level surfaces with younger ages (e.g., KL1 581 and PT2) have an unrecognized burial history short-term incision along the Chandra River is locally very high, reaching 12 mm/a. This rate is 631 notable, since the Chandra is cutting down through quartzo-feldspathic crystalline bedrock. 632 Bedrock strength (erosivity of bedrock) exerts a critical control on the incision rate of bedrock 633 channels River between Batal and Koksar, and the variability of its incision within this stretch indicate 636 that it is still in post-glacial adjustment to Lahul's tectonically active landscape, where hillslope 637 mass movements appear to be the dominant mechanism of erosion (Owen et al., 1995). 638 639 Our calculated incision rates along the Chandra River reflect differential incision over 640 time and the length of the river. There appears to be a lag of ∼ 5 ka between the retreat of the 641 main glaciers that reached into the Chandra Valley and fluvial bedrock incision, although it is 642 possible that older, higher terraces have been destroyed or buried by mass wasting events. The 643 age data illustrate the variation that is possible in Himalayan river incision over spatial and 644 temporal scales. This again highlights the varying amounts of incision that are possible over time 645 and space in this active Himalayan environment. 646 647 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT Although our sampling area is relatively small, there is a clear contrast in our incision 648 data with rates and patterns of river incision in other actively uplifting mountains at convergent 649 plate margins that can be interpreted in terms of steady-state landscape evolution (e.g., Pazzaglia 650 and Brandon, 2001). This contrast suggests that Lahul's landscape is in disequilibrium, and 651 given the relatively modest long-term exhumation rate, further suggests that disequilibrium has 652 been persistent on timescales of 10 6 years. 653 654 655 Conclusions 656 657 Our AHe ages show that exhumation of the HHCS in Lahul from shallow crustal levels to 658 the surface is very young, occurring during the past ∼ 2.5 Ma. Even if the uncertainty in our 659 AHe measurements could be reduced -uncertainty largely due to low U concentrations -this 660 conclusion would not change. Our AHe ages also fit into the regional pattern of low-temperature 661 thermochronometers in the HHCS yielding Plio-Pleistocene ages. The largely igneous bedrock 662 in Lahul along the Chandra valley and its tributaries was exhumed from cool, presumably high 663 crustal levels at the same time that rocks in other regions of the HHCS -where there is 664 evidence for active Quaternary faulting and rapid fluvial erosion -were being exhumed from 665 hotter crust where isotherms were likely telescoped near the surface. Surface exposure ages on 666 some strath terraces more than 10 m above the contemporary river level are as ≤1.5 ka. 667 Calculated incision rates along the Chandra are as high as 12-13 mm/a. Thus, on the million-668 year timescale that typically governs isotope thermochronometers, comparison of AHe ages 669 highlight variations in the near-surface thermal structure of the Himalaya that have developed