T41C-01 INVITED
Timescales of sedimentation and erosion of an intermontane basin fill: geomorphic processes in the Quebrada del Toro basin, NW Argentina
Externally drained intermontane basins in tectonically active mountain belts often act as areas of sediment storage that buffer the transfer of sediment from the orogen interior to the foreland. As such, they may serve as important recorders of the deformation and erosion within orogens. The timescales associated with sediment filling and excavation of such basins during several glacial cycles (>105yr) remain mostly unknown, but are of significant importance when analyzing sedimentary records in light of climatic vs. tectonic forcing and with respect to landscape evolution in active orogens. The northwestern Argentine Andes consist of numerous fault-bounded basins that document a rich record of cycles with transient closure/reduced fluvial connectivity, coeval conglomerate deposition and subsequent re-integration of the fluvial network accompanied by efficient erosion processes. Specifically, the Quebrada del Toro is an exceptionally well-documented intramontane basin in NW Argentina whose long-term depositional history records the tectonic uplift of downstream ranges and the progressive isolation of this basin from the foreland fluvial system. Herein, we place coarse bounds on the age of these terraces using 20 new cosmogenic nuclide ages that quantify basin filling and excavation during the middle Pleistocene (<0.98m.y.) in the Quebrada del Toro basin within the Eastern Cordillera of NW Argentina. We have dated 4 terraces with cosmogenic nuclides (10Be, 26Al) that record the deposition and removal of ~ 60km3 of material related to inhibited or perhaps transiently halted erosional removal of material from the orogen. We have collected several samples from depth profiles from each terrace level to constrain the exposure history of each fill level, which unconformably overlies an old fill unit that onlaps older deformed and eroded strata. The distribution of exposure ages shows that reworking of exposed material from higher terrace levels constitutes a significant component of the cosmogenically-derived exposure ages and we note that transient storage and recycling of material has played a major role in terrace formation in this environment. A preliminary assessment of our surface-exposure ages does not show a correlation with major climatic cycles recorded in other archives in the Andes, suggesting that local effects exert an important control on the sedimentary evolution of this basin. We speculate that a combination of sediment supply controlled by climatic cycles superposed on tectonic activity at the range front ultimately results in the formation and evacuation of intermontane basin fills. Interestingly, prior to an 0.98m.y. alluviation event, the base-level of the Quebrada del Toro river was already cut to the present-day river level, underscoring the importance of transient sediment storage of basins bordering the Altiplano-Puna Plateau. Similar observations apply to many intermontane basins of variable size that straddle the eastern margin of the Puna Plateau and may also apply to other mountain belts with similar structural and climatic characteristics.
T41C-02
Relationships Between Climate and Erosion over Long and Short Time Scales in the High Himalayas
Numerical and conceptual models that invoke feedbacks between surface and tectonic processes make the common assumption that higher rainfall drives higher erosion rates. This assumption was tested in the High Himalayas of Nepal by monitoring the suspended sediment flux of rivers that span a steep rainfall gradient. The suspended sediment flux data were combined with solute data and a bedload:suspended load ratio to estimate watershed-scale erosion rates. We find a positive relationship between erosion rate and rainfall. In fact, we find that a catchment on the southern flank of the Himalayas has the highest erosion rate and the greatest rainfall, and it also forms the hanging wall of the Main Central Thrust. These results would seem to support the existence of feedbacks between climate, erosion, and tectonics. The modern day erosion rates, however, are at odds with the rates measured over 100,000 - millions of years which imply uniform erosion over the region. We propose that rapid glacial erosion in the rainshadow of the Himalayas during the Ice Ages compensates for the relatively slow erosion during the interglacials.
T41C-03
Steady-state relief in the Southern Alps of New Zealand during the last glacial cycle.
Glaciers and rivers control the shape of the high relief topography of most modern mountain ranges. Their relative contribution in response to climatic oscillations and whether landscape can reach steady state conditions during the Quaternary is still unclear. We present here how we have developed a new thermochronometric method based on optically stimulated luminescence (OSL)-dating, that we call OSL- thermochronometry. This system has a closure temperature low enough (25-40°C) to enable quantification of both the exhumation rate and change of topography at the time-scale of a glacial cycle (ca. 100~kyr). We apply the technique to a young, tectonically active mountain belt where geomorphic conditions have oscillated between glacial and fluvial-dominated systems for most of the Quaternary, the Southern Alps of New Zealand. Our findings imply that the drainage basin relief has reached a form of steady state over at least the last glacial cycle. This implies that, despite the extreme exhumation rate of the order of 500 to 1000 m in 100 kyr, and the fact that, in the last 11-18 kyr, most valleys have switched from U- to V-shape and have been re-incised and dissected by debris-flows, rock avalanches and landslides, the relief amplitude did not change at the wavelength of a glacial valley (i.e. ca. 8~km).
T41C-04
Climatic and tectonic controls on deposition of the Punaschotter conglomerate in Neogene marginal basins of the Puna Plateau (NW Argentina): Evidence from zircon U-Pb geochronology
Sedimentary sequences within basins along and within the southern margin of the Puna Plateau, Northwest Argentina, record the completing influences of tectonics and climate on exhumation and deposition in this dynamic region. The basins contain Mio-Pliocene fluvial-lactustrine strata, which are capped by distinctive proximal alluvial-fluvial conglomerates collectively referred to as the Punaschotter conglomerate. Previous geochronology suggested an approximately concurrent onset of Punaschotter conglomerate deposition in each basin, despite their different positions within the orogen, pointing to a climate driven origin. However, new zircon U-Pb microprobe ages from intercalated ashes from the Fiambalá, El Cajon and Angastaco basins, combined with existing data from the Corral Quemado and Santa Maria basins and the Antofalla basin within the plateau demonstrate the onset of Punaschotter deposition at different, non- overlapping times with no clear spatial distribution, with ages ranging from ~7 to 2.5 Ma. A direct regional climatic control on the onset of Punaschotter conglomerate deposition, such as onset of the monsoon precipitation pattern or glaciation in the Andes, is therefore unlikely. A regional structural cause, such as uplift resulting from removal of mantle lithosphere beneath the Puna Plateau, would also produce a simultaneous effect in each basin, and so can be ruled out. Local structural uplift is an alternative explanation; however, the onset of deformation adjacent to each basin seems to predate the onset of Punaschotter deposition. Furthermore, AFT and AHe thermochronology indicates less than 3 km exhumation of surrounding ranges in the late Miocene to Pliocene. Overall, the data favor a scenario in which a convergence of regional climate change and aridification and local uplift creates the conditions for deposition of the Punaschotter conglomerate at different times in each basin. Thus climate and tectonics act in concert to control deposition and erosion around the margin of the Puna plateau.
T41C-05
Spatial Gradients of Precipitation and Erosion in the Swiss Alps: Direct Evidence and Mechanistic Explanation for the Glacial Buzzsaw
The glacial buzzsaw hypothesis predicts that glacial erosion, mediated by the ELA (equilibrium line altitude) controls alpine maximum topography. Spatial variability in precipitation along the southern margin of the Swiss Alps is pronounced with a doubling of precipitation over spatial scales of 50 km occurring in the Ticino region. We predict the ELA across the Swiss Alps using climatological precipitation patterns based on long gauge records and observed temperature lapse rates. 502 cirques from the southern Swiss Alps are identified from digitized 1:25,000 scale topographic map contours. We also measured the relief of peaks above cirque floors. Local peak elevation is strongly correlated with cirque-floor elevation (r = 0.81). Precipitation has a strong inverse correlation with cirque-floor elevation (r = 0.58) as well as a strong inverse correlation with peak elevations (r = 0.52). The dependence of ELA on precipitation gradients is determined independently of local topography, and it is found to decrease by 520 ± 75 meters per meter of precipitation. This precipitation-driven ELA lowering matches the observed decrease of cirque-floor elevation of 616 ± 39 meters per meter of precipitation and the observed decrease of peak elevation of 507 ± 37 meters per meter of precipitation. Thus, the observed and predicted impact of precipitation on glacial erosional features and ELA are within error of one another indicating that glacial erosion, as modulated by the ELA, is a fundamental control on alpine topography.
T41C-06
Thermochronological Evidence for a Late Pliocene Climate-Induced Erosion Rate Increase in the Alps
(U-Th)/He and fission-track analyses of apatite along deep-seated tunnels crossing high-relief mountain ranges offer the opportunity to investigate climate-tectonic forcing on topographic evolution. In this study, the thermochronologic analysis of a large set of samples collected along the Simplon railway tunnel (western- central european Alps) and on its surface trace constrains in detail the mechanisms controlling the topographic evolution of the Simplon Massif and the timing in which they operated. The study area is located at the western margin of the Lepontine metamorphic dome where a complex nappe-stack pertaining to the Pennine realm experienced a fast exhumation at 18-15 Ma. The exhumation was mainly accommodated by a west-dipping low-angle detachment (the Simplon fault) which is located just 8 Km to the west of the analyzed section. However, along the section itself several faults belonging to two principal sets (NW-SE and ENE- WSW) both with an important dip-slip kinematic component have been detected. Cooling rates derived from our thermocronological data vary from about 10°C/Ma at about 10 Ma to about 35°C/Ma in the last 3 Ma. Such increase in the cooling rate corresponds to the most important climatic change recorded in the northern hemisphere in the last 10 Ma, i.e. the inception of glacial cycles in the northern hemisphere. In addition, (U-Th)/He and fission-track age patterns show an important correlation with the mapped faults until 2 Ma, suggesting that tectonics controlled rocks exhumation up to that age. After 2 Ma, no fault-controlled tectonic activity has been recorded by thermo-chronometric data, hence the entire Simplon area has experienced primarily erosional exhumation. All thermochronologic age patterns determined in this study are not affected by topographic effects. More in detail, the (U-Th)/He tunnel ages show an impressive uniformity at 2 Ma, whereas cooling rates calculated at 1 Ma increase towards the two major valleys. This indicates a focusing of the erosive processes in the valley which have led to the shaping of the present-day topography.
T41C-07
A closer look at the Neogene erosion and accumulation rate increase
Glacial erosion and Quaternary cold-stage warm-stage climate cycling have been cited as mechanisms to explain observations of increased Neogene marine sedimentation rates. Quantification of long-term glacial erosion rates from cosmogenic radionuclides from large areas mostly covered by cold-based ice during the Quaternary show very low erosion rates over several glacial cycles. In addition, isotope ratio proxies of dissolved metals in seawater, measured in chemical ocean sediments, lack clear evidence for an increase in terrigenous denudation. In particular, the stable isotope 9Be, derived from continental erosion, shows no change in its ratio to meteoric cosmogenic nuclide 10Be, derived from rain over the past 10 My. Radiogenic Pb and Nd isotopes, mainly show a change in the style of denudation from more chemical to more physical processes in the Quaternary. These data are at odds with a suggested increase in marine sedimentation rates during the late Cenozoic. In order to resolve this contradiction we have scrutinized these sedimentation rate calculations from ocean cores to identify whether they might show only apparent increases in the Neogene sections. Potential explanations are that in some cases, measured sediment thicknesses for different time intervals lack corrections for sediment compaction. Compaction of the lower portions of the cores drastically increases the apparent thickness of the more recent (Quaternary) sediment. In addition, sedimentation rates often only appear higher for recent sections in cores due to an artifact of an averaging timescale that decreases up-core. Such an averaging time scale decrease arises from better chronological resolution in recent times (Sadler et al., 1999). Cannibalization of older sediment might add to this effect. Together, these data question a clear, global-scale Quaternary climate-erosion connection that would be unique in Earth's history.
T41C-08 INVITED
Climate Forcing of the Growth and Destruction of the European Alps
At the scale of an orogen, climatic and tectonic processes operate together to produce and form topography and to modulate the proxy signals that reflect that topography, including cooling ages and sediment yield. However, in terrestrial settings, there are few unambiguous climate proxies and disentangling climatic and tectonic forcing of geologic data is difficult. Small convergent orogens such as the Alps are particularly interesting as products of tectonic and climatic forcing as they are frequently in a state of near-balance between accretionary and erosional fluxes, so that perturbations can have a large effect. Models of the coupling between tectonic crustal deformation and climate-modulated surface erosion provide insight into how we might best differentiate between tectonic and erosional forcing, predicting a relationship between mountain belt height, width, sediment yield and exhumation rate. In particular, positively or negatively correlated changes between sediment yield and orogen width can be used to identify climate forcing. Two major events stand out in late Alpine history as potential climatically-forced events. The period from 30 to 20 Ma, characterized by little outward growth of the Alps, but high rates of exhumation and sediment yield, was terminated at the Aquitanian, Burdigalian boundary. At this time, the Alps expanded southward into their foreland, accompanied by a decrease in sediment yield both north and south of the Alps. This can be interpreted as a climatically-driven change from a steady to a constructive state. The second major event occurred at 5 Ma, when southward expansion of the Alps ceased, and the northern foreland inverted and began to erode, both characteristics of the transition to a destructive state. This transition was accompanied by a large increase in sediment yield, supporting the hypothesis that this was also a climatically-driven change.