T12D-01
Orogenic Plateaux Produce Sigmoidal River Profiles
Rivers draining an orogen control the transport of sediment through it. A tenet of fluvial geomorphology is that a river's longitudinal profile evolves towards a graded, concave-up form. Curvature analysis of river long profiles, using satellite images and SRTM data, shows this to be incorrect for the major rivers draining the Tibetan and Turkish-Iranian plateaux. Instead, both plateaux show long wavelength (>100 km) convexities in rivers on a regional scale. These convexities cause the rivers to have sigmoidal long profiles, with low gradient reaches at high elevations. We suggest that the control on the convex-up reaches is the combination of the lack of active thrusting and aridity in the plateau interior. Both factors result in low levels of incision at high elevation. Simple numerical modelling shows variation in incision along the length of a river can produce profiles that are similar to those observed. Limited incision within a plateau interior suggests a lower sediment flux from orogenic plateaux than if a graded profile were attained.
T12D-02
The role of crustal strength variations in shaping collisional orogens
Continental crust is often composed of regions with different lithologies, histories, thermal structures, and pre- existing anisotropies. Such large-scale variations in crustal properties are likely to play an important role in defining the mode and localization of crustal deformation in a continental collision zone. Using a three- dimensional semi-analytical numerical model of deformation in a viscous crust, we investigate the effects of lateral heterogeneities in the strength of both the upper and lower crust on the patterns of uplift and surface deformation in collisional orogens. The model includes a two layer crust and allows for flow of a weak lower crust as well as lateral and temporal variation of viscosity in both the upper and lower crust. Model results indicate that variations in crustal strength have a dramatic effect on the morphology and dynamics of a developing orogenic plateau, and influence not only the location and steepness of plateau margins, but also the transport of material throughout much of the plateau. As a developing plateau encounters regions of strong crust, the adjacent plateau margin becomes steep, remains localized along the boundaries of the strong crustal block, and is commonly concave in map view. These margins do not accommodate significant shortening strain, and velocities in both the upper and lower crust are generally directed subparallel to the plateau margin. In contrast, a weak crustal region develops a gently sloping margin that propagates rapidly across the weak zone and accommodates large amounts of shortening. Crustal material within the plateau moves towards the low- strength region, with rapid flow towards and across the low-gradient plateau margin. The contrasting styles of deformation and morphology that accompany these lateral strength variations enable us to recognize crustal strength variations in actual orogens. The morphology of the northern and eastern margins of the Tibetan plateau are consistent with the presence of strong crust in the regions of the Sichuan and Tarim Basins and weak crust beneath the southeastern margin. With a relatively simple distribution of strength variations, our model is able to reproduce much of the large-scale topography and surface motions of Tibet, particularly in eastern Tibet. Analysis of model results and plateau morphology suggests that variations in crustal strength have played a fundamental role in shaping the plateau and have likely influenced the development of major structures such as the Altyn Tagh and Xianshuihe Faults.
T12D-03
Recent Evolution of the Central Longmen Shan (Eastern Tibet) documented by denudation processes
Located west of the Sichuan Basin, the Longmen Shan range constitutes the eastern border of the Tibetan Plateau. It presents a steep regional topographic gradient, comparable with what is observed in the Himalayas. Nevertheless, despite this important topographic step, no significant active convergence can be detected across that border of the Plateau. Based on those observations several authors have proposed alternative explanations for topography building and sustainment in this area. The most widely advocated model propose the existence of crustal channel flow of low viscosity material from the Plateau interior toward the Sichuan Basin (e.g. Clark et al, [2005]). This flow is supposed to be deviated by the cold rigid Yangtze Craton under the Sichuan Basin, which induces surface uplift in the Longmen Shan and would explain the existence of high topography in this area. We present a new set of data combining (U-Th)/He thermochronometry, CRN measurements in alluvial sands, and quantitative geomorphology. Our data indicate that, (1) maximum denudation is not coincident with the high topographic front, but is located behind this area, and (2) the intensity of erosion decreases with time. From those observations we propose that the temporal and spatial evolution of denudation processes for the last 10 Myr can be associated with the propagation of a regressive erosion wave on a tectonically passive inherited topography, and that channel flow is not necessarily a prevailing mechanism for the evolution of this region. The global implications of this scenario are evaluated using finite element thermo-mechanical modelling. We test the response to erosional unloading of a topographic step, without tectonic forcing. the output of those modellings are consistent with available data (heat flow, geodetic velocities, erosion pattern), and it appears that the action of erosion on the Plateau margin induces large scale deformation, inside the Tibetan crust.
T12D-04
Slip rates of active thrusts and rates of river incision in the Qilian Shan (NE-Tibet)
The Qilian Shan is the north-easternmost of several mountain chains, which constitute a several hundred kilometer wide intra-continental fold-and-thrust belt in NE-Tibet (1). Spectacular, up to 50-m-high fault scarps at the front of the Qilian Shan and well preserved geomorphic surfaces allow to quantify slip rates of active thrust faults using surface exposure dating. In order to constrain vertical uplift rates of dip-slip faults it is crucial to separate tectonically-controlled river incision from non-tectonic incision, which is ultimately related to climate change. This is best achieved at sites where fault scarps are transacted by small rivers and tectonic offsets determined from scarp profiles can be compared directly to the amount of river incision recorded by fluvial terraces. Detailed investigations of several thrust faults that offset alluvial fans and river terraces, or have tilted large pediments yielded Late Quaternary fault slip rates that are relatively low, i.e. 0.4 to 1.3 mm/yr (2-4). At each studied site, the amount of river incision is similar to the vertical fault displacement. In contrast, post-glacial incision of large rivers at the Qilian Shan mountain front is predominantly climate-related and resulted in steep valleys cut into Late Quaternary conglomerates. At the Shiyou He, for instance, climate-induced Holocene river incision proceeded at a rate of about 10 mm/yr, thus exceeding the Late Pleistocene incision rate - which may largely be controlled by tectonic forcing - by a factor of ten (5). References: (1) Meyer et al. (1998), GJI 135, 1-47; (2) Hetzel et al. (2002), Nature 417, 428-432; (3) Hetzel et al. (2004), Tectonics 23, TC6006, doi:10.1029/2004TC001653; (4) Hetzel et al. (2004), Terra Nova 16, 157-162; Hetzel et al. (2006), JGR Earth Surface 111, F03012, doi:10.1029/2005JF000352.
T12D-05
Middle Miocene exhumation of the Laji Shan, Northeastern Tibetan Plateau
New data from northeastern Tibet uplift and associated basin tectonic evolution provides constraints on the mechanics of plateau growth and tectonic/climate interactions related to the aridification of central Asia and the intensification of the east Asian and Indian monsoons. Evolution of the large Longzhong Basin - including the Xining, Lanzhou, Linxia, Gonghe and Guide sub-basins - is well-constrained by recent advances in magnetostratigraphic dating of climatic and tectonic records. In particular, aridification is associated to global cooling at the Eocene-Oligocene transtion 34 yrs ago (see Hoorn et al., this meeting, Session T17), and tectonic activity is associated to widespread clockwise rotations of paleomagnetic directions at ca. 41 Ma and 17-11 Ma (see Dai et al., this meeting, Session GP12). However, poorly constrained thermal histories in adjacent fault- bounded ranges make linkages between tectonic, exhumation/sedimentation and climate tentative. We present here preliminary apatite (U-Th)/He and Fission track (FT) ages from the Laji Shan, a mountain range presently separating the Guide-Gonghe basins from the Xining-Linxia-Lanzhou basins. Sampling was performed at three elevation profiles (MD, LJ and TG;) in plutonic rocks from 2068 m to 4114 m. (U-Th)/He results from the lower elevation profile MD transect indicate fast exhumation between 14 Ma to 8 Ma, while the higher elevation profiles LJ and TG results indicate widespread ages from 40 to 110 Ma typical of partial retention zone. Obtained apatite FT dates of the MD elevation profile range from 16 to 35 Ma and all samples have long Mean Tracks Lengths (MTL) indicating fast cooling through the partial annealing zone for all samples. There is a clear correlation between elevation and FT date. The data indicate that a fossil partial zone has been uplifted. The sample at the lowest elevation (2068 m) with an apatite FT age of 16.2 Ma has the highest MTL suggesting that rock uplift of some 6 km has taken place since 16 My ago, assuming a geothermal gradient of 25°C/km. Obtained apatite FT datesof the LJ elevation profile cluster around 60 ± 10 Ma and do not differ much over the entire sampled elevation range. MTL are shorter for samples from the high elevation part of the profile, compared to longer MTL from the lowest samples. This is interpreted in reflecting a fossil partial annealing zone that has been uplifted since 55 My ago, as indicated by the sample with the longest MTL of 14 μ and FT age of 52.4 Ma. The total amount of rock uplift since that time amount to 6-7 km (assuming 25°C/km). Obtained apatite FT dates from the TG elevation profile cluster around 130 ± 10 Ma. The FT data reveal a fast cooling of the rocks around 160-170 Ma ago and all samples seem to have passes through the partial annealing zone in a similar way, suggesting a continuous exhumation of the rocks. Taken together these data suggest that significant midlle Miocene (ca. 15 Ma) unroofing in the Laji Shan belt can be associated to compartmentalization of the large Longzhong basin with tectonic rotation and onset of sedimentation in the Gonghe-Guide basins. http://www.geo.uu.nl/~forth/people/Guillaume
T12D-06
Deposition and Deformation in the El Cajon Basin, NW Argentina: a record of climate change, plateau growth and foreland fragmentation
The series of intermontane basins located along the southern margin of the Puna Plateau in NW Argentina are uniquely positioned to record a variety of surface, structural and geodynamic processes. Deformed strata in the basins document foreland fragmentation and out of sequence reverse faulting in a thick-skinned setting with pre- existing crustal heterogeneity. The basins also show evidence for closed or restricted outlets and times of filling, which could have led to incorporation into the morphologic plateau. However, the basins have been re-integrated, perhaps multiple times, reflecting both climate and tectonic controls. Finally, the deposition of the Punaschotter conglomerate in late Pliocene or Quaternary time could reflect local tectonic control or regional climate change. In this study we focus on the El Cajon basin, which contains ~1500 m of Miocene through Quaternary sedimentary strata. These strata have been deformed by reverse faulting to the west (Sierra Chango Real) and uplift of a basement cored anticline (Sierra de Quilmes) to the east, and are cut by several mostly east-vergent faults within the basin. Growth strata document syn-depositional deformation. We present new structural mapping and U-Pb dating of 13 intercalated tuffs from the tertiary stratigraphy. These data, in addition to three existing ages, allow detailed determination of the rate of deposition over time, the timing and extent of a lacustrine interval within the basin, the depositional age of the Punaschotter conglomerate, and a maximum age constraint on the onset of basin incision. Combined with our structural analysis, and with data from the other marginal basins, we can assemble a regional picture of basin fragmentation and tectonic and climatic controls on deposition and deformation around the margin of the Puna Plateau.
T12D-07
Changing Styles of Deformation Along the Western Margin of the Andean Plateau, Southern Peru
The Cordillera Occidental in southern Peru provides an important record of the deformational signature of the Andean Plateau formation. Early deformation along the western margin of the developing plateau was primarily accommodated along a series of high-angle, west-vergent reverse faults. This fault system crops out in a 100-km wide region from the Cordillera Occidental to the Peruvian coast. Although several research teams have attributed much of the construction of the western plateau margin to these features, a wide variety of high-angle faults and broad monoclines also occur along this margin. Understanding the relative significance of these features to plateau margin development has been hampered by limited of constraints on the timing and magnitude of fault movement, as well as uncertainty about the history of surface uplift. Structural mapping and thermochronologic studies in southern Peru provide new insights into the timing and magnitude of late Cenozoic surface uplift as well as the style of structural accommodation. The west-vergent reverse fault system deforms Oligocene sediments and older bedrock, but is covered by 14-16 Ma volcanic units. More recent faulting close to the range front is extensional in nature and has been an important contributing factor to the formation of the modern range-front escarpment. These structures deform the 14-16 Ma volcanic units, but most show only limited offsets. Valley-bottom transects of thermochronologic data from Cotahuasi valley show no major discontinuities in age, indicating that offset on any of the structures cannot exceed several hundred meters. Farther to the east, normal fault movement has not been as tightly constrained, but empirical relationships between fault length and maximum throw suggest at most 1 km of movement, or 500 to 800 m of vertical offset considering a reasonable range of fault dips. We combine these observations with new apatite (U- Th)/He data from vertical and valley-bottom transects of samples that provide robust constraints on the timing of surface uplift. These data suggest that the major phase of 2 to 2.5 km of plateau uplift that started at c. 10-12 Ma cannot have been solely accommodated by surface-breaking fault movement. Instead, regional long-wavelength warping and possibly a component of block uplift dominated the late Cenozoic deformation history of the western margin.
T12D-08
Sensitivity of Denudation Rates to Latitudinal and Orographic Variations in Climate, Central Andes, Bolivia.
Continental denudation rates are commonly thought to be sensitive to both tectonics and climate, but the importance of each effect is difficult to constrain. In this study we present 17 new cosmogenic 10Be data to quantify medium-term, basin-averaged denudation rates from the Andean fold and thrust belt, Bolivia. Sampling occurred across two different transects in both the northern (~-15°S) wet and southern (~- 19°S) dry Subandes. These data are compared with (1) modern latitudinal variations in precipitation, (2) orographically controlled E-W climate gradients, and (3) differences in morphological parameters. In addition, results are used to (4) verify temporal variations in denudation rates by comparing millennial-scale (cosmogenic nuclide-derived) and decadal-scale (sediment yield-derived) rates. Cosmogenic nuclide-derived denudation rates are as follows: In the southern transect denudation rates vary by two orders of magnitude and range between 0.04 to 2.20 mm/yr over timescales between 0.4-19 kyr. The highest values and largest variability in denudation rates are observed in the smallest catchments with drainage basin areas <60 km2. In the northern transect denudation rates range between 0.13 and 0.95 mm/yr over timescales between 0.5-6.8 kyr. Here, the highest values are observed in small basins located along the frontal range of the Andean mountain chain. Interestingly, the mean value for the cosmogenic nuclide-derived denudation rate is 0.57 +/- 0.07 mm/yr for the dry south and 0.35 +/- 0.04 mm/yr for the wet north. In contrast, modern denudation rates calculated from suspended sediment flux data suggest that average rates in the south (0.3 mm/yr) are 3-4 times slower than in the north (1.3 mm/yr). Previous results exhibit strong spatial and temporal variations in denudation rates and lead to the following implications: (1) Cosmogenic nuclide-derived denudation rates are poorly to uncorrelated with drainage basin characteristics such as relief or slope. (2) An obvious relationship is observed between denudation rates for catchment sizes >1000 km2 and precipitation. (3) Denudation rates in small catchments (<200 km2) do not show a statistical relationship with climate. Climatic effects seem to be superimposed by tectonic influences (e.g. proximity to active faults). (4) Temporal variations in denudation rates might be related to an anthropogenic increase in sediment yield in areas with intense farming. Another hypothesis is that climate changed during the integrated time interval and cosmogenic nuclide-derived denudation rates inherited a past climate signal.