T11G-01
Stable Isotopic Constraints on Climatic and Topographic Development Along the Northern Margin of the Tibetan Plateau
One of the salient features of the greater Himalayan/Tibetan orogen is the large sedimentary basins that sit along the northern margin of the Tibetan Plateau. These basins record the surface uplift of adjacent ranges and the attendant reorganization of climatic patterns. As such, we present a record of oxygen and carbon isotope values of Cenozoic sedimentary units along the northern margin of the Tibetan Plateau. The 15 sedimentary sections that this study encompasses have been measured and sampled at the meter scale, and include strata of fluvial, lacustrine, floodplain, and alluvial fan origin, as well as deposits of one brief marine transgression. Localities studied include sections in the Qaidam Basin (Xiao Qaidam, Lake Mahai, Lao Mangnai, Ganchaigou, Lenghu, and Lulehe), Tarim Basin (Puska, Aertashi, Miran River, and Jianggalsay), Hexi Corridor (Shiyougou, Jiuquan, and Baiyanghe), and the Altun Shan (Subei and Xorkol). These sections were chosen because they are the most temporally complete nonmarine Cenozoic sections in the region and have the most robust age constraints, which include biostratigraphy, magnetostratigraphy, and thermochronology. Many of the same sections have also been previously used to decipher the structural history of nearby ranges. An initial subset of the >3,000 samples collected agrees with the previous findings of Graham et al., 2005, but our increased spatial and temporal resolution reveals how isotopic stratigraphic records locally reflect development of topography and subsequent reorganization of atmospheric circulation. For example, in the Qaidam Basin, two stratigraphic sections in the northern part of the basin show that oxygen isotope values increase steadily by 5-6 permil from the middle Eocene to Pliocene, despite several changes between fluvial and lacustrine depositional environments. This contrasts with the isotope stratigraphies from western Qaidam presented in Graham et al. (2005) and this study, that show a steady oxygen isotope profile between the Oligocene and Miocene, followed by a decrease in oxygen isotope values between the Miocene and Pliocene. The variation in isotopic records shows that the isotopic expression of the development of topography may vary widely within a basin depending on study locale. Isotopic analysis of modern waters that we and others have collected in this region is consistent with this interpretation, with rivers recording distinct values locally depending on their drainage area. The importance of local bounding ranges controlling the isotopic evolution of these sections is confirmed by our paleocurrent and provenance work.
T11G-02
Stable Isotope Paleoaltimetry of Evolving Orogenic Plateaus
High-elevation orogenic plateaus and mountain ranges exert a strong control on global climate and precipitation patterns and respond to tectonic processes in the lithosphere and upper mantle. Reconstructing the history of surface elevation thus provides a critical link between erosive, climatic, and tectonic processes. Stable isotope studies of lacustrine, fluvial and pedogenic environments, intermontane and intra foreland basins, as well as core complex-bounding detachment faults record the Cenozoic isotopic and sedimentologic fingerprint of the evolving landscape of the North American Cordillera. Stable isotope paleoaltimetry allows the determination of long-term elevation changes, yet complex terrestrial precipitation and hydraulic patterns necessitate integrative approaches, comprising various isotope systems and mineral proxies. Combined stable isotope, geochronological, sedimentological, and structural data suggest that during the early Eocene the western edge of Cordilleran orogen consisted of a proto-Sierra Nevada that continued into a broad plateau of the Sevier hinterland bordered on its eastern flanks by intra foreland basins with local basement uplifts. Following Mesozoic shortening, topography and regional mean elevation were spatially and temporally transient. Stable isotopic and sedimentological data suggest that a landscape characterized by increased peak elevations and elevated relief migrated from northeast to southwest as the landscape became high and more rugged. This occurred between 50 and 47 Ma in SW Montana, between 40 to 35 Ma in northern Nevada, and by ~22 Ma in southern Nevada. Embedded in the context of stable isotopic studies in terrestrial basins across and along strike of the Cordillera, we see an emerging picture of major readjustments in surface elevation and relief combined with reorganization of continental drainage systems and envisage a landscape that first responds to mid-crustal flow followed by normal faulting in crustal-scale detachment systems.
T11G-03 INVITED
Tectonics, climate and surface processes of the southern central Andes: insights from stable C isotopes and sedimentary environments
The Puna of the southern central Andes comprises a high-elevation, semi-arid to arid low-relief, which forms an integral part of the Altiplano orogenic plateau. Internally, the plateau is compartmentalized into sedimentary basins and intervening basement rages that were uplifted in a diachronous manner throughout the present plateau region, similar to ongoing deformation and uplift along the current plateau margin in the broken foreland. A unifying feature of the Puna-Altiplano plateau is that its location corresponds to sectors of the landscape in which channels have generally failed to incise deeply into basin sediments or through surrounding basement ranges. Importantly, in both areas the local base-level is hydrologically isolated from the foreland. This isolation occurs where the incising power of regional drainage systems has been greatly reduced due to a combination of diminished precipitation related to regional climate and local orography, and exposure of bedrock that is resistant to fluvial incision. This hydrologic isolation from the foreland restricts evacuation of eroded material, consequently leading to internal drainage and a reduction of relief between basins and the surrounding peaks. Thus, while a variety of deformation styles and possibly combinations of different processes may have generated the high elevations and structural compartmentalization associated with the plateau, the low-relief character of the plateau may be a geomorphic, rather than a tectonic phenomenon. Basins similar to those in the Puna exist along the plateau margin, although these basins remain only transiently isolated and internally drained due to the proximity to high precipitation gradients which were established due to orographic barriers related to Pliocene uplift. These barriers focus precipitation and erosion and promote headward erosion, stream capture and ultimately basin exhumation and connection to the foreland, which prevents these areas to become incorporated into the plateau realm. The fact that humid environments exist at the eastern plateau margin seems paradoxical at these latitudes (22-27°S), as this region is inherently dry. New U-Pb chronostratigraphy on intercalated volcanic ashes in sediments exposed in the Subandes of Bolivia and analysis of paleosols in NW Argentina and Bolivia using stable C isotopes reveal that the onset of humid conditions along the eastern flanks of the southern central Andes was at approximately 8 Ma. We suggest that enhanced precipitation in this region was closely linked to the evolution of the Puna-Altiplano plateau and its adjacent eastern orographic barriers that forced the southward displacement of easterly moisture-bearing winds via the Low Level Andean Jet. In conclusion, by late Miocene time the orogenic system of the southern central Andes must have attained critical elevation thresholds to form an effective orographic barrier and rainfall originating from the dynamics of the South American Monsoon must have started to affect these latitudes.
T11G-04
Orographic barriers, high-resolution TRMM rainfall, and relief variations along the eastern Andes
The complex interplay between erosion and tectonics shapes landscapes on various time and length scales. Thus, determining the relative importance of tectonic and climate-driven processes is key for our understanding of the evolution of tectonically active mountain belts. Each mountain belt may exhibit an individual and complex response to these processes that may be characteristic for differently sized sectors of the orogenic system. Here, we focus on small (101km) to medium (102-103km) scale relations along the eastern flank of the meridionally oriented South American Andes. The central Andes and the major part of the northern Andes are subject to strong rainfall and erosion on their eastern side. There, rainfall has a strong gradient with wet, northern and central parts and presently drier areas south of the Andes bend at ~17°S. While lithology and geologic units vary along strike, the general deformation regime of the northern and central eastern Andes is similar with east vergent thrust-fault systems. We use high-resolution Tropical Rainfall Measurement Mission (TRMM) and SRTM topographic data to characterize elevation, relief, and hillslope angle of peak rainfall at orographic barriers constituting the eastern flank of the orogen. Over a distance of more than 3500km along the eastern flank of the Andes facing the Amazon Basin, we find that peak rainfall (>3.5m/yr) occurs at a mean elevation of 1.3±0.17km, a mean 3- km-relief of 0.95±0.08km, and areas characterized by moderate mean hillslope angles of 18.3±1.7°. We find that topographic relief is the best first-order rainfall predictor. South of the eastward-convex bend of the Andes, rainfall amounts decrease and there exists no distinct, high rainfall peak. The reduction in rainfall is accompanied by a reduction of relief involving several hundred meters at the mountain front as rivers exiting the eastern Andes tend to form large sedimentary fan systems that reduce the distance between minimum and maximum elevation. For ~500km south of the bend, the mean hillslopes of the eastern Andes maintain moderate values before they decrease significantly. We relate this reduction to a change in deformation style from east to more west vergent fault systems at ~24°S due to the compressional reactivation of inherited Cretaceous extensional fault systems. This, in turn, results in slow, gently westward-rising hills with steep western sides. The lack of a pronounced high-relief zone on the eastern, windward side does not create a focused rainfall peak that exceeds the necessary threshold amounts for many erosive processes. In summary, there are two key results of our work that will help understand the shaping of mountain belts through erosive and tectonic processes: First, high rainfall peaks with amounts above 3.5m/yr in the northern and central eastern Andes exceed the triggering mechanisms for many erosive hillslope processes. There, hillslopes are moderately steep despite varying lithology and deformation-rate changes. Second, a minimum 3-km-relief of ~1km is needed to create a significant rainfall peak. This relief threshold is very constant for 3500km along strike of the orogen and suggests relief to be one of the controlling factors for peak rainfall formation in this environment. The lack of high-relief barriers on the eastern Andes in southern Bolivia and NW Argentina prevents the formation of a rainfall peak that in turn would lead to an area with focused erosion.
T11G-05
A Slow and Steady Growth of the Andean Plateau?
Quantifying the timing and rates of central Andean mountain building and plateau formation have previously been limited by (1) sparsely available age constraints on the timing of deformation, and (2) a lack of constraints on the elevation history of the plateau. Recent thermochronometer and paleoaltimetry studies from the plateau and adjacent fold and thrust belt have started to address these limitations. In particular, paleoaltimetry results suggest a rapid 3.7 +/- 0.4 km increase in plateau elevation between 10.3-6.7 Ma, based on stable and clumped isotope, and paleobotanical techniques. This interpretation is based on the assumption that the paleoclimate at the time the paleoaltimetry proxies were deposited was similar to modern. In this study, we evaluate the influence of changing Andean plateau height on South American paleoclimate and interpretations of plateau elevation from paleoaltimetry data. A series of experiments are presented using regional (RegCM) and Global (GENESIS) General Circulation Models (GCM) to characterize changes in Andean precipitation amount, surface temperature, and wind direction (vapor source) as a function of changing plateau elevation. Results indicate that South American and Andean climate changed significantly in response to plateau growth. More specifically, lowering of the plateau to below 0.50-0.75 of its present day elevation results in an 700 mm/yr decrease in mean annual precipition over large portions of the Andes. Plateau lowering also results in 6 to 10 C increase in temperatures (after correction for a change in lapse rate) over the central Andean Plateau. Finally, the prevailing wind direction and the vapor source for precipitation progressively changes from the equatorial Atlantic to South Pacific as plateau elevation decreases. Taken together, these changes in paleoclimate would have enriched the oxygen isotopic concentration of precipitation by an estimated -11.4 to - 16.6 per mil, and resulted in an apparent paleo-elevation that is up to several kilometers lower than the true elevation at the time of rainfall. We conclude that the apparent rapid rise of the Andean Plateau from climate sensitive paleoaltimetry data may be an artifact of large changes in paleoclimate. Thus, the simple model of a slow and steady rise of the Andean Plateau associated with crustal thickening may be viable.
T11G-06
Deformation and Exhumation History of the Central Andes, Bolivia: Insights From Thermochronology and Numerical Modeling
The central Andean plateau is a principal feature of the Andes with a debated uplift history. In Bolivia, geologic evidence has resulted in several proposed end-member plateau uplift scenarios. These scenarios include distributed or eastward-propagating deformation beginning in the late Eocene to late Oligocene (~40-27 Ma) and recent rapid surface uplift of ~2.5-3.5 km during the late Miocene (~10.3-6.7 Ma). We synthesize 52 new thermochronometer samples with model-predicted cooling ages for the previous scenarios. Predicted and observed ages are compared to quantify the exhumation and deformation history across the eastern Andean plateau margin and test the uplift scenarios. We used inverse modeling of apatite fission-track data to resolve sample cooling histories and variations in the onset and duration of Andean deformation. Results from data collected across the Bolivian thrust belt in both the north and south suggest rapid exhumation began (a) during multiple episodes in the late Eocene through Miocene (~40-10 Ma) in the Eastern Cordillera (EC), (b) in the middle to late Miocene (~20-15 Ma) in the Interandean zone (IAZ), and (c) in the Mio-Pliocene (~20-2 Ma) in the Subandes (SA). We used a 2D thermokinematic finite-element model to predict sample exhumation histories and cooling ages. Predicted fission-track ages are cooling-rate dependent and a function of imposed end-member kinematic and erosion histories. A simplified topographic profile of the plateau margin is applied over multiple time steps to simulate plateau growth. Model free parameters include the temporal evolution of topography, thermal boundary conditions, material properties, and the kinematic field. Model results suggest: (a) erosion rates are ~0.1- 0.4 mm/yr in the EC/IAZ and ~0.01->1 mm/yr in the SA, and (b) cooling age variations are sensitive to the plateau uplift history because uplift is coincident with accelerated erosion and sample exhumation. The protracted uplift scenarios predict more complicated along-strike age variations comparable to the observations at >~0.2 mm/yr background vertical exhumation rates. Work in progress is evaluating the sensitivity of these results to all model free parameters.
T11G-07
What controls the asymmetry of convergent mountain belts?
Mountain belts formed along convergent plate boundaries often exhibit a significant topographic and structural asymmetry that is correlative with the polarity of subduction and accretion. In general, the side of the orogen on top of the down-going plate, or the pro-side, tends to be wider and less steeply-tapered than the retro-side of the orogen, which is often characterized by a crustal-scale backfold. This asymmetry seems to be relatively invariant of the exact plate kinematics and of orogen-scale climatic influences (e.g., rainshadow). In this work, we investigate the controls on orogen asymmetry through complementary suites of numerical and analogue simulations, restricted to the frictional deformation of the Earth's crust. The analogue experiments demonstrate that asymmetry prevails after an initial symmetric phase of deformation and that growth of the retro- side occurs at a much later stage of orogen evolution. Comparison of analogue experiments with and without isostatic compensation suggests that isostasy significantly increases orogen asymmetry. This result is also observed in the numerical experiments, which are used to investigate the range of behavior between no isostatic compensation and local isostatic compensation. The numerical experiments also demonstrate that the contrast between the internal and basal strengths has an important, although secondary, influence on orogen asymmetry, with higher basal strengths tending to enhance asymmetry. As a whole, our results suggest that structural and topographic asymmetry observed in natural systems is primarily driven by isostasy and not by the polarity of subduction and accretion as commonly believed.
T11G-08
Erosional Reduction of an Orogenic Wedge: Structural Response to Neogene Climate Change within the St. Elias Orogen, Alaska
The kinematics and architecture of orogenic systems may be heavily influenced by climate, but little research has focused on the long term effects of glacial erosion on orogenesis. Apatite and zircon (U-Th)/He thermochronometry on >75 bedrock samples across the St. Elias orogen, one of the best examples of a glaciated orogenic wedge, is the basis for a new kinematic model and demonstrates an association between glacial denudation and orogenic architecture. The spatial pattern of low temperature cooling indicates that exhumation and deformation are focused within a thin-skinned fold and thrust belt on the windward flank, whereas the leeward flank of the orogen functions as a deformational backstop. A previously unrecognized structure beneath the Bagley ice field must separate these domains with south-side-up motion. We propose this structure is a backthrust making the orogen doubly-vergent. Suggestive of accelerated backthrust motion in response to climate change, cooling rates within the hanging wall block and across the entire windward flank of the orogen accelerated ten-fold coeval with enhanced glaciation. As backthrust motion increased, glacial unroofing also coincided with a regional shift in deformation away from prominent forethrusts including the North American-Yakutat terrane suture (Chugach St. Elias fault) and the seaward deformation front (Pamplona zone). Across the windward flank of the orogen, exhumation, at rates of up to 5 mm/yr, is focused within a narrow zone, where the glacial equilibrium line altitude (ELA) intersects the orogenic wedge. This zone of rapid exhumation, not present prior to the onset of enhanced glaciation, cuts across the structural trend of the orogen and is more narrowly focused than orographic precipitation. Accelerated denudation at the ELA thus appears to have redistributed strain along a series of forethrusts that lie at the zone of heaviest glacial flux, while the backthrust progressively truncates the southward-vergent forethrusts. In a cause and effect response, the expansion of glaciers therefore appears to have resulted in a narrowing of the orogenic wedge due to increased backthrust motion and a landward propagation of deformation in order to preserve topographic slope. This focusing of long- term glacial erosion and deformation at the ELA matches predictions from analytical models of orogenic wedges (i.e. Tomkin and Braun, 2002) and implies a high degree of coupling between climate and tectonics in this glacially-dominated orogen.