TECHNICAL COMMENT Comment on ‘‘Slip-Rate Measurements on the Karakorum Fault May Imply Secular Variations in Fault Motion’’ Chevalier
et al. (1) presented cosmic-ray exposure dates for glacial deposits offset by movement along the Karakorum Fault. They inferred a late Quaternary slip rate on this fault, 10.7 T 0.7 mm/year, that is higher than rates reported in recent field studies (2). The reported rate is also greater than present-day rates based on interferometric synthetic aperture radar (InSAR) analyses (3) and Global Positioning System (GPS) measurements (4), a finding that led Chevalier et al. to conclude that slip on the fault varies over time scales longer than the recurrence interval between earthquakes. Clasts within the two studied moraine systems had cosmic ray exposure ages ranging from 21 to 45 thousand years (ky) (n 0 9, mean 0 35.6 ky, 1s 0 8.7 ky) and from 103 to 325 ky (n 0 18, mean 0 177 ky, 1s 0 63 ky). The two moraines were assigned ages of 21 T 1 ky and 140 T 5.5 ky, respectively. We question the approach of Chevalier et al., however, for selecting the most accurate date for moraine deposition from a scattered group of ages for individual clasts incorporated in a moraine. Any postdepositional process that affects the exposure history of individual clasts (for example, burial, erosion, spalling, and shifting position) will reduce cosmogenic nuclide accumulation and apparent exposure ages and increase scatter in the data (5, 6). Moraine boulders thus often show tightly grouped exposure ages in young surfaces (7) and wider scatter in stratigraphically older formations. Nevertheless, in certain cases, scatter in exposure ages for a given landform results primarily from variable exposure before deposition in current positions; this has been noted in small alluvial and debris-flow fans (2, 8). In such systems, exposure ages of some clasts will greatly exceed the time of landform deposition. Chevalier et al. (1) adopted this view in evaluating their data set. In contrast, if material deposited on a landform has been exhumed from depths great enough to minimize previous exposure to cosmic radiation, any scatter will be the result of postdepositional processes. Under these conditions, exposure ages un-
derestimate the age of the feature with which they are associated; the clast with the highest cosmogenic nuclide concentration will most closely reflect the actual landform age. The data of Chevalier et al. provide some basis for evaluation of which of these views more closely represents the processes leading to the observed age distributions. If scatter in apparent ages were due to previous exposure, moraines of similar size and morphology deposited by the same glacier (for which there would be no reason to expect fundamentally different glacial regimes) should show comparable absolute ranges of scatter in their ages. Ages for the younger moraine show far less scatter (1s 0 8.7 ky) than those for the older moraine (1s 0 63 ky), which suggests that variation in prior exposure is an unlikely explanation for the dispersion of dates observed for the older moraine. Furthermore, incision and modification of the older surface Esee figure S2 in (1)^ indicate the influence of surface disturbances that diminish apparent exposure ages. The ages that Chevalier et al. propose for deposition of the two moraines correlate with the coldest episodes of the SPECMAP climate curve (9). However, this chronology is inconsistent with other regional studies that indicate that little glacial expansion occurred during the last glacial maximum (LGM) and that the greatest glacial expansion within the last glacial cycle was considerably earlier, at È40 thousand years ago (ka) (10–12). This growing body of literature suggests that alpine glacial expansion often depends on regional processes, such as moisture transport, and thus does not necessarily coincide with growth of Northern Hemisphere continental ice sheets. For the younger moraine, Chevalier et al. proposed that the age indicated by the majority of the samples (È40 ka, based on seven of nine samples) represents an earlier advance but that the subsequent glacial event (È21 ka, based on two of nine samples) is correlative with the actual termination of moraine deposition. This scenario is not consistent with the proposed slip rate. If the moraine had been deposited over the
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20-ky period between 40 ka and 20 ka while the fault was moving at a rate of 10 mm/year, the material should have been spread across a 200-m lateral span, with the youngest material closest to the valley. This is not the case; there is no systematic spatial pattern of Byoung[ and Bold[ clasts Esee figures 2B and 3B in (1)^. Evaluating the data under the assumption of minimal previous exposure yields minimum ages of È325 and È45 ky and maximum slip rates of È4.7 and È4.9 mm/year for the older and younger surfaces, respectively. Such rates would corroborate the results of previous work that directly dated offset geomorphic markers along the Karakorum fault (2). In addition, they would be consistent with recent InSAR (3) and GPS (4) analyses and thus would not require hypotheses of large fluctuations in fault motion over time. Erik T. Brown Large Lakes Observatory and Department of Geological Sciences University of Minnesota Duluth Duluth, MN 55812, USA E-mail:
[email protected] Peter Molnar Department of Geological Sciences and Cooperative Institute for Research in Environmental Science University of Colorado at Boulder Boulder, CO 80309, USA Didier L. Bourle`s Centre Europe´en de Recherche et d’Enseignement en Ge´osciences de l’Environnement Unite´ Mixte de Recherche 6635 CNRS and Universite´ d’Aix-Marseille III Europoˆle de l’Arbois F-13545 Aix-en-Provence cedex 4, France References 1. M. L. Chevalier et al., Science 307, 411 (2005). 2. E. T. Brown et al., J. Geophys. Res. 107, doi: 10.1029/ 2000JB000100 (2002). 3. T. J. Wright, B. Parsons, P. C. England, E. J. Fielding, Science 305, 236 (2004). 4. S. Jade et al., Geol. Soc. Am. Bull. 116, 1385 (2004). 5. J. Putkonen, T. Swanson, Quat. Res. 59, 255 (2003). 6. B. Hallet, J. K. Putkonen, Science 265, 937 (1994). 7. J. C. Gosse, J. Klein, E. B. Evenson, B. Lawn, R. Middleton, Science 268, 1329 (1995). 8. J. Van der Woerd et al., Geology 26, 695 (1998). 9. J. Imbrie et al., in Milankovitch and Climate, Part I, A. Berger, J. Imbrie, J. Hays, G. Kukla, B. Saltzman, Eds. (Reidel, Boston, 1984), pp. 269–305. 10. L. A. Owen et al., Geol. Soc. Am. Bull. 115, 1356 (2003). 11. R. C. Finkel, L. A. Owen, P. L. Barnard, M. W. Caffee, Geology 31, 561 (2003). 12. D. I. Benn, L. A. Owen, J. Geol. Soc. 155, 353 (1998). 21 March 2005; accepted 1 July 2005 10.1126/science.1112508
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TECHNICAL COMMENT metric synthetic aperture radar (InSAR)– derived rate (È0 mm/year) of Wright et al. (8), and the recent Global Positioning System (GPS) results (3.4 T 5 mm/year) of Jade et al. (9) but in agreement with the GPS results (11 T 4 mm/year) of Banerjee and B[rgmann (10) (Fig. 1A). The disparity among the rates may indicate secular variation in the slip rate. Brown et al. (1) argue that the slip-rate disparity is not real and that we have systematically underestimated the ages of the Brown et al. (1) argue that the dispersion in ky). The resulting slip rate is È10 mm/year— offsets. They contend that the younger moraine was emplaced during MIS 3, which, based on greater than the geomorphic rate (È4 mm/year) the exposure ages we obtained from the glacial chronologies in the Himalayas, as determined by Brown et al. (7), the interferoManikala glacier moraines (2) is influenced opposed to Tibet proper, would by postdepositional processes and 20 have been the period of greatest that only the oldest boulder age on A glacial expansion during the last each moraine group can be used glacial cycle (11, 12). However, to assess its abandonment age. LGM advances of large Tibetan Determining the appropriate age 15 glaciers like the Manikala, although of a geomorphic offset from a Chevalier et al.(2), M2E restricted, have been documented dispersed age population does Banerjee and Bürgmann (10) (13). Moreover, the history of indeed present a considerable 10 glaciation on the plateau is variable. challenge, especially where dates Chevalier et al.(2), M1 Owen et al. (14) have dated LGM from surface samples are not and post-LGM moraines in the supported by subsurface sampling, Jade et al. (9) western Nyainqentanglha Shan at radiocarbon dating, or climatic 5 Brown et al. (1) Karola Pass (three glacial advances correlation. In general, predeposiyounger than 20 ky) and in the tional exposure will yield ages Wright et al. (8) Right-Lateral Gongar Shan (four advances younthat are too old, and postdepo0 ger than 10 ky). sitional processes will yield ages Left-Lateral At Manikala, two LGM-age that are too young. samples—with ages that are statisThe slip rates determined by -5 tically distinct from all others found Chevalier et al. (2) were based on 1 10 100 1000 10 4 10 5 10 6 on M1—support the contention that the surface exposure ages of two Age (yr) glacial expansion during the LGM composite moraine groups that was sufficient to allow the glacier were displaced from their source to cross the fault. The age of the by right-lateral motion on the 1600 B offset may thus be approximated by Karakorum Fault. The younger M2 the youngest ages, yielding the moraine deposits yielded surface 1400 higher rate. The distribution of 18 exposure ages ranging from 21 to 1200 ages from the older moraine has 45 thousand years (ky), and the peaks at È140 and È180 ky, older moraine yielded ages from 1000 r consistent with cold periods during 103 to 325 ky. On either moraine, /y m m 800 and at the end of MIS 6. Two of the the frequency of ages displayed 2 . 9 18 samples yielded ages of È315 distinct modes that correspond to 600 and È325 ky, forming another the coldest epochs of the last two small, statistically distinct cluster glacial cycles—which, contrary to 400 M1 10.7 ± 0.7 mm/yr that is only slightly younger than the assertion of Brown et al. (1), is 200 the age of the glacial maximum just unlikely to be coincidental. We yr / mm prior to MIS 9.3 (È340 ky). The argued (2) that the offset moraine 5.5 0 200 150 small number of È325 ky ages edges only became passive offset 100 50 0 relative to those between 140 and markers when the glaciers had Age (ka) 180 ky and the similarity of these retreated, and inferred the age of Fig. 1. (A) Comparison of slip-rate determinations with observation interval the offsets to be those of the from geodetic methods (open symbols) and geomorphic methods (filled ages to a previous glacial maximum younger peaks in the distribution, symbols). (B) Rates obtained from the offset age relations for the Manikala suggest that they represent deposits which correlate with global and moraine complex. An average rate of 10.7 T 0.7 mm/year is obtained from of a previous glaciation that were regional climate records (3–6): The linking the 1520 T 50 m and 220 T 10 m offsets, with the end of MIS 6 carried across the fault during È1500-m offset thus accumulated (È140 ka) and MIS 2 (È20 ka), respectively. This association yields a subsequent glacial advances. They since the end of Marine Isotope constant slip rate over the entire observation interval. A slip rate of 5.5 mm/ would thus represent an inherited year is implied if the 220 T 10 m is linked with the 40 T 3 ky age of the older Stage 6 (MIS 6, È140 ky) and the subgroup on M1. This low slip rate over the past È40 ky requires a rate of component, such as that docuÈ220-m offset since the last 9.2 mm/year between È40 ka and È180 ka to reconcile the rate obtained if mented in most of the cases where glacial maximum (LGM, È20 the 1520 T 50 m offset is linked with the 181 T 14 ky age cluster on M2E. enough individual boulders are www.sciencemag.org
.9 ± 10 0.6 .5 m ± m 0. /y r 5 m m /y r
10
Of fset (m)
Slip-rate (mm/yr)
Response to Comment on ‘‘Slip-Rate Measurements on the Karakorum Fault May Imply Secular Variations in Fault Motion’’
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TECHNICAL COMMENT dated to characterize the surface age distribution Eusually up to 10 or more, e.g. (15, 16)^. As for the younger moraine, the age of glacial retreat, 140 thousand years ago (ka), determines the age of the offset. The extent to which surface reworking influences the apparent surface exposure ages of glacial moraines is not solidly established. Using surface exposure and radiocarbon dating of alluvial terraces and moraines along the Altyn Tagh fault in northwest Tibet, M2riaux et al. (17) demonstrated a constant slip rate (linear offset versus age) for the last È120 ky. Erosion, surface reworking, or both will preferentially Byoung[ the exposure ages of the oldest samples, resulting in a nonlinear relation between offset and age in which apparent slip rate increases with time. That this is not observed suggests that the most important process in degrading glacial surfaces may be headward stream erosion and fluvial incision of the original glacial surface, which is easily identified in the field and on satellite images and which was absent where we sampled at Manikala. Our study (2) considered the possibility that the younger moraine was abandoned at È36 ka and the older moraine at È180 ka (the older peak in the age distribution). This offset-age assignment results in a rate of 5.5 mm/year between 36 ka and the present and a rate of 9.2 mm/year between 180 ka and 36 ka (Fig. 1B), well in excess of the InSAR rate over much of the observation interval. This scenario, however, requires a major change in the long-term slip rate without plausible tectonic justification. Extrapolating a lower rate of 4 to 5 mm/year back in time would yield an age of 9350 ky for the È1500-m
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offset recorded in the older moraines. If that were the case, one would have to explain the highly unlikely scenario in which the prominent cold periods during MIS 6 produced no discernible glacial record, whereas other cold maxima did. Arguing that the ages of È140 and È180 ky from the older moraine represent this record is, in essence, the interpretation of Chevalier et al. (2). Given the inconsistencies inherent in alternative models, we believe that the glaciotectonic history and derived slip rates we present (2) provide the simplest interpretation of the overall distribution of ages. Whether there is disparity between millennial and decadal rates will depend as much upon future geodetic measurements as it does upon our confidence in the geomorphic determinations. M.-L. Chevalier Laboratoire de Tectonique Me´canique de la Lithosphe`re Unite´ Mixte de Recherche (UMR) 7578, CNRS Institut de Physique du Globe de Paris 75252 Paris cedex 05, France E-mail:
[email protected] and Institute of Geophysics and Planetary Physics Lawrence Livermore National Laboratory Livermore, CA 94550 USA F. J. Ryerson Institute of Geophysics and Planetary Physics Lawrence Livermore National Laboratory P. Tapponnier Laboratoire de Tectonique Me´canique de la Lithosphe`re UMR7578, CNRS Institut de Physique du Globe de Paris
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R. C. Finkel Institute of Geophysics and Planetary Physics Lawrence Livermore National Laboratory J. Van Der Woerd Institut de Physique du Globe de Strasbourg CNRS-UMR 7516, France Li Haibing Laboratory of Continental Dynamics Institute of Geology Chinese Academy of Geological Sciences Beijing 100037, China Liu Qing Total Exploration China Total-Fina-Elf Beijing, 100004, China References 1. E. T. Brown et al., Science 309, 1326 (2005); www. sciencemag.org/cgi/content/full/309/5739/1326b. 2. M. L. Chevalier et al., Science 307, 411 (2005). 3. J. Imbrie et al., in Milankovitch and Climate, Part I, A. Berger, J. Imbrie, J. Hays, G. Kukla, B. Saltzman, Eds. (Reidel, Boston, 1984), pp. 269–305. 4. D. G. Martinson et al., Quat. Res. 27, 1 (1987). 5. L. G. Thompson et al., Science 246, 474 (1989). 6. L. G. Thompson et al., Science 276, 1821 (1997). 7. E. T. Brown et al., J. Geophys. Res. 107, doi: 10.1029/ 2000JB000100 (2002). 8. T. J. Wright, B. Parsons, P. C. England, E. J. Fielding, Science 305, 236 (2004). 9. S. Jade et al., Geol. Soc. Am. Bull. 116, 1385 (2004). 10. P. Banerjee, R. Bu¨rgmann, Geophys. Res. Lett. 31, article no. 1652 (2002). 11. L. A. Owen et al., Geol. Soc. Am. Bull. 115, 1356 (2003). 12. R. C. Finkel, L. A. Owen, P. L. Barnard, M. W. Caffee, Geology 31, 561 (2003). 13. L. A. Owen et al., Z. Geomorphol. 47, 263 (suppl.) (2003). 14. L. A. Owen et al., Quat. Sci. Rev., in press. 15. D. C. Douglass et al., Geology 33, 237 (2005). 16. J. Putkonen, T. Swanson, Quat. Res. 59, 255 (2003). ´riaux et al., J. Geophys. Res. 109 (B6), article 17. A.-S. Me no. B06401 (2004). 13 April 2005; accepted 27 July 2005 10.1126/science.1112629
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