Todd A. Ehlers - US grants

Quantifying Glacial Erosion Rates, Magnitudes, and Paleotopography in the Coast Mountains, British Columbia

P.I. Todd A. Ehlers - University of Michigan
EAR-0309779

In the last 12 years a global scientific debate has ensued over if climate change or tectonics provided the impetus for the appearance, or reality of accelerated mountain uplift. This three year project at the University of Michigan will test the hypothesis that cooler climates, and more specifically glaciation, increase the topographic relief and maximum height of mountain ranges, thereby producing the appearance of accelerated uplift in the last ~10 Million years. Previous attempts to test this hypothesis have been hampered by a lack in understanding of the distribution and amount of glacial erosion over million-year time scales. This study will integrate geochemical data (low-temperature thermochronometers) with three-dimensional computer models to quantify the glacial erosion history in the southern Coast Mountains, B.C.
The integration of new data collection and modeling for this study will involve several steps. (1) Two seasons of fieldwork will provide 60 new samples within a ~25x25 km area of the southern Coast Mountains. (2) Two years of computer program development are needed to modify an existing three-dimensional computer model to simulate evolving topography. The model will predict exposed sample ages under a variety of tectonic, and erosional scenarios. (3) The last two years will apply an inverse computer-modeling program to determine the range of glacial erosion histories, and ancient preglacial topographies, that satisfy the measured geochemical sample ages. The results from these three steps will test the stated hypothesis by reconstructing the mountain topography prior to glaciation in the Coast Mountains and assess if glaciation increased topographic relief.
In addition to addressing a long-standing problem in the scientific community, this study will also provide a Ph.D. for one graduate student. The principle investigator and graduate student will give regular talks in K-12 school science classes on 'glaciers and ice ages' and at least one undergraduate student will be involved, and mentored in the research process.

Understanding the timing, rates, and deformation associated with mountain belt plateau formation is central to quantifying relationships between plateaus, plate tectonics, and climate. Computer models of plateau development invoke variations in crustal temperatures and strength as the main mechanism of plateau formation. Strength variations are important because as tectonic processes shorten and thicken the crust the lower crust weakens and flows laterally, preferentially supporting plateau formation over a narrow mountain belt. However, tests of these computer models are hampered by a lack of knowledge about the kinematic history of plateau formation. This project is using rock cooling histories, field-constrained structural analysis, and computer models to delineate the tectonic evolution of the Bolivian fold and thrust belt to infer the formation process and surface uplift chronology of the Andean Plateau.

Apatite fission track (AFT) and zircon (U-Th)/He (ZHe) ages are being collected and analyzed along two transects in the Bolivian fold and thrust belt to determine rock cooling histories. The AFT and ZHe data are being used to define the timing and rates of uplift along each sampling transect. Coupled two dimensional thermal, tectonic, and erosion computer models are being used to quantify the history of the Bolivian fold and thrust belt by predicting cooling ages and comparing them with the AFT and ZHe data. The coupled computer models aid in the interpretation of data through several steps: (1) the tectonic history of the thrust belt is prescribed using the 2DMove software, restored structural cross-sections, and geologic maps along both profiles, (2) the tectonic model then drives advective heat transfer in the thermal model and surface uplift in the erosion model, (3) for each tectonic and erosion history simulated across the fold and thrust belt the thermochronometer ages exposed at the surface are computed using apatite fission track annealing and zircon He diffusion algorithms. Model predicted and observed ages are compared with statistical methods to identify the range of permissible tectonic and erosion histories for Andean Plateau formation and surface uplift. The data and computer models are being integrated to quantify the timing, rate, and duration of motion on each thrust fault across each sampling transect. Combining the deformation and erosion history of all the faults across each transect will provide constraints on (1) the timing and history of Andean Plateau formation, and (2) temperature and strength dependent computer models of plateau formation.

A workshop entitled "Thermochronology" is being held in Snowbird, Utah in October of 2005. Workshop participants are: The two primary aims of this workshop are the (1) assessment of the state of the art of thermochronology and the identification of future potential and outstanding problems in the field, and (2) education of students and early career scientists in methods, concepts, and applications of thermochronology. Participants are attending lectures by leading experts in the field, extended discussion sections, and hands-on computer software training sessions. Workshop participants include graduate students, early career scientists, and scientists from underrepresented groups in the geosciences. Workshop results will be disseminated broadly through publication of a special volume by the Mineralogical Society of America.

PROJECT SUMMARY

Quantifying changes in erosion and relief with detrital apatite (U-Th)/He thermochronlogy and cosmogenic nuclides

Technical Description:
This study performs a detailed study of the distribution and rates of catchment erosion in the southern Sierra Nevada of California using integrated detrital apatite (U-Th)/He thermochronometry and cosmogenic nuclides in sediment. Our first line of research proposes that detrital apatite (U-Th)/He cooling ages can act as sediment tracers, and thus be used to test whether a suite of steep catchments, in order of increasing size and complexity, are eroding uniformly or are dominated by point source erosion processes. Our second line of research compares the distribution of cooling ages from modern river sediments with those from older deposits, specifically river sediments preserved in caves, to investigate temporal changes in relief and catchment hypsometry. We use a 3D thermal model to account for the influence of the topographic bending of isotherms on interpreted detrital grain-age distributions from sediment. In addition, we propose to use cosmogenic 10Be and 26Al concentrations in river and cave sediments to investigate spatial and temporal changes in catchment-average erosion rates. Detrital apatite (U-Th)/He thermochronometry and cosmogenic nuclides are easily integrated tools because they utilize different minerals from the same bag of river sand. By developing and integrating these tools in detrital settings, and by exploiting a wealth of previous (U-Th)/He data and a set of well-dated caves, we investigate the topographic evolution of the southern Sierra Nevada, California, from the earliest Pliocene to today. Our integrated thermochronometric and cosmogenic approach is readily applied to other settings. For example, our development of the detrital apatite (U-Th)/He approach will determine the suitability of this technique for older deposits (e.g., sedimentary basins) for quantifying paleorelief and paleoerosion rates in orogenic belts around the world.

Broader Significance:
This project addresses fundamental problems in evolution of the Earth's surface in a mountain range of keen interest to geologists and laypersons alike. Our research should help answer how steep mountain valleys in the Sierra Nevada erode, the rates at which they erode and produce sediment, and how the distribution of elevation within valleys has evolved in light of climate change (e.g. repeated glaciations) over the last 2 million years. Furthermore, the geochemical tools we will develop should prove valuable in the overall effort of quantifying topography and erosion rate changes in mountain belts over longer timescales (>2 million years).
Broader significance to the scientific community include development of a new application of a geochemical tool (apatite (U-Th)/He thermochronometry) and integration of this technique with other more conventional geochemical tools (e.g. cosmogenic isotopes). Career development and training associated with this project includes training and preparation of a postdoctoral scientist for a career in academia. The study will also facilitate at least one undergraduate student completing a senior thesis associated with this project. Public outreach and K-12 education will occur in the form of (1) education of National Park Service interpreters at Sequoia-Kings Canyon National Parks charged with teaching park visitors about the geology and landscapes of the southern Sierra Nevada, and (2) involvement of an elementary school teacher to development an awareness of current research topics in the Earth sciences as well as new lesson plans and course materials.

Erosion in mountain belts results from the complex interplay between tectonic and climatic forces. In order to assess how these forces interact to control orogenic erosion, it is essential to establish a detailed record of the long-term erosional history of mountain belts. The thermochronologic ages preserved in syn-orogenic sediments reflect cooling during exhumation of the source terrane and therefore detrital studies represent a powerful tool with which to establish a record of mountain erosion. Most investigations utilize a single thermochronometer to assess the cooling history preserved in a population of grains. However, the individual grains in a sediment or sedimentary rock do not necessarily share a common thermal history and were likely derived from different areas within a mountain belt that may erode at distinct rates. This exploratory, field-oriented study is focused on developing an alternative approach that uses paired thermochronologic ages from syn-orogenic conglomerate deposits. Because all grains in a single cobble share a common thermal history, paired dating reveals the detailed time-temperature path taken by the sample during exhumation. These thermal histories are used to constrain the erosional history of the source terrane. This approach is being tested in a small area of the Spanish Pyrenees, where the Sierra de Sis conglomerate body preserves a 20 million year history of syn-orogenic deposition. The cooling histories from several samples throughout the stratigraphic section are being measured to hypothesize how the rate of erosion in the mountain belt may have evolved throughout the lifespan of the orogen. To investigate how variations in the erosion rate may be associated with pulses of faulting, the thermochronologic results is also compared with new estimates on the timing of local thrusts obtained from the dating of fault gouge.

This research holds significant promise for enhancing earth scientists' understanding of the long-term evolution of mountains. A major question in contemporary tectonics is how tectonic and climatic forces interact to control the shape and structure of mountain belts. This exploratory study is seeking new tools to obtain the detailed and long-term records of erosion necessary to address this problem. Once established, this approach may be widely applied to reveal the erosional history of orogenic belts worldwide, including ancient settings where all that remains of a mountain belt are its eroded remnants. In addition, it will support an early career post-doctoral scientist.

The grandeur of mountain topography has for millennia captured the attention of poets, artists, and scientists. How plate tectonic processes of mountain building and mountain erosion by surface processes interact to produce topography over millions of years is now at the forefront of Earth science research. A fundamental question that arises when studying the evolution of mountains is: what did the past topography of mountain ranges look like? This question has proven very difficult to answer. Recent developments in both computer modeling of mountain building and erosional processes, and developments in geochemistry have made progress in reconstructing paleotopography. Advances in new geochemical techniques and mathematics (inverse problem theory) now allow a means of testing computer model predictions with geochemical (thermochronometer) data from rocks exposed at the Earth's surface today. These data record the cooling history of rocks as they are exhumed to the surface by erosion and faulting. This interdisciplinary project is addressing questions and hypotheses that are fundamental to quantifying the evolution of mountain topography including: (1) How can geologically meaningful interpretations of tectonic and geomorphic processes influencing mountain topography be improved from an integration of thermochronometer data, computer modeling, and mathematics? (2) How sensitive are thermochronometer data to different mountain building and erosional processes and how can sampling strategies be optimized to improve interpretations? and (3) What is the magnitude and rate of topographic change that can be resolved from mathematical inversion of thermochronometer data?

To address these questions, this project investigates the forward and inverse problems of mountain topographic evolution with a comprehensive model. Coupled 3D thermal, hydrologic, and kinematic computer models are under development in addition to a surface process model accounting for glacial, fluvial, and hillslope erosional processes. The coupled model is used to explore the sensitivity of thermochronometer data to different processes and mathematically invert a dense network of new and existing thermochronometer samples from the southern Coast Mountains, B.C., for the regional paleotopography. Field work is in progress for the collection of additional data. Several novel mathematical techniques are also under development. In particular, a low pass filter technique and a regularized iterative method are being used to solve the notoriously ill-posed backward parabolic equation and large scale, nonlinear inverse heat transport equation. These problems are by nature interdisciplinary and in the forefront of predicting and interpreting thermochronometer data and mountain topography.

The formation of the Andean Plateau is not well understood partly because the timing and rates of plateau uplift are poorly constrained. Estimates based on paleoclimate and thermochronometer data are consistent with formation of a plateau of modern width by about 13 million years ago. In contrast estimates based on stable isotope paleoaltimetry, a relatively new technique that employs the oxygen isotope concentration (delta-18O) of ancient soil carbonates to infer past elevation change, suggest a rapid uplift about 10 to 7 million years ago. Isotope paleoaltimetry is a promising new tool for quantifying elevation change, but several outstanding issues may complicate its interpretation. In particular, changes over the last 40 million years in surface temperature, atmospheric circulation, precipitation rate, and vapor source may affect the oxygen isotopic concentration of ancient soil carbonates. If substantial, these effects could compromise paleoelevation inferences based on the isotopes The objective of this project is to evaluate the processes that control delta-18O of precipitation in the Andes and to test the hypothesis that Andean Plateau uplift was in fact steady over the last 20 million years. The apparent rapid rise of the plateau in the late Miocene may be an artifact of changes in Cenozoic climate and atmospheric circulation that caused depletion of Andean precipitation 18O. Global and regional climate models with the capability to predict oxygen and deuterium isotope transport and fractionation provide a tool for quantifying these effects over the Andean Plateau. However, before these models can be used with any confidence, they must be validated against modern observations of oxygen isotopes in precipitation, which are currently sparsely available in the Andes. In this project, a multi-year sampling campaign across central Peru and southern Bolivia measures monthly variations in the oxygen isotope composition of precipitation. These data, when integrated with existing measurements, meteorological data, and backtracking analyses will provide critical data for assessing climate model predictions of precipitation oxygen isotopic composition and evaluate whether a Rayleigh law adequately predicts isotope fractionation on the Andes. The validated regional and global climate models are used to predict past changes in Andean delta-18O composition and to assess the influence of past climate changes on precipitation delta-18O used in soil carbonates for paleoaltimetry studies.

The Andean Plateau is one of the most dramatic topographic features on Earth. Despite its impressive nature, the mechanisms and rates of its formation remain poorly understood. A range of competing geodynamic models have been proposed to explain the formation of plateaus. To distinguish between these models it is necessary to understand the deformation and erosion history associated with the plateau and its marginal thrust belts, the present day crustal and lithospheric structure, and the paleoelevation history of the plateau. Of these, current understanding of Andean paleoelevation is arguably the most uncertain, and yet offers great insights into the formation of the plateau. The paleoelevation history can be used to make inferences about the rate and timing of mountain building, and ultimately the geodynamic processes that govern mountain building. The results of this project will provide an important evaluation of the isotope paleoaltimetry technique and provide improved calculations of the surface uplift history of the Andean Plateau.

This is an ambitious project that has the potential to fill in important gaps in the overall picture of orogenesis in the central Andes, and of convergent-margin tectonism in general. The project is constructed around a well defined basic-science question, did the Andes rise in a rapid pulse, or did they rise gradually? Producing elevations and crustal thicknesses of the magnitude found in this study area remains a key problem in continental tectonics.

This question provides a foundation from which the PIs develop a variety of linked projects, including: 3-D structural analysis of fold-thrust belt shortening in the Andes, testing of new methods of paleo-elevation analysis, use of seismic studies to characterize the roots of the range (both in the deep crust and in the underlying mantle), creative use of petrologic and isotopic data to constrain thickened crust at times in the past. The project has the potential to address 3-D mass balance issues during orogeny, as well as the impact of a rising mountain belt on continent-scale weather systems. Of note, to put the analysis of orographic weather studies in context, the PIs will also undertake a broader paleo-climate study. All of the questions to be studied are current and important, and are of interest across traditional disciplinary boundaries and, the research strategy as outlined has a high potential to answer the questions that it poses.

Previous studies of stable isotopic paleoclimate proxies found in intermontane basins and adjacent metamorphic core complexes suggest that the topography of western North America developed diachronously, obtaining high elevations first in British Columbia at about 50 million years ago and sweeping into Nevada by about 40 million years ago. The stable isotopic studies show that there are rapid and large isotopic shifts that cannot be due to surface uplift alone and call for climatic controls. This research aims to test the hypothesis that relief development and possibly regional scale surface elevation (driven by tectonics) attained threshold values that caused rapid climate and precipitation shifts by actively interfering with atmospheric vapor transport and/or stability. To test this hypothesis, the research team is using a multi-disciplinary approach that involves: (1) collection of stable isotopic data from intermontane basins over discreet time intervals and over a wide geographic area so as to compare with isotope results from climate models; (2) measurement of cooling ages of detrital minerals in an effort to constrain relief and mountain building development within the basin catchments; (3) detailed sedimetological and high-resolution geochronologic studies in basins in order to place the detrital thermochronology and stable isotopic analyses in proper geologic context; and (4) simulation of climate conditions and isotopes of precipitation under different topographic/elevational scenarios using global and regional climate models as a way to interpret the observed stable isotope signals. The goal is to discriminate between two markedly contrasting tectonic models both of which are consistent with current data sets. One calls for the construction of dynamic topography from a moderate elevation low-relief landscape to a north-to-south swell of a high elevation landscape in the Eocene to Oligocene. The other is the north-to-south collapse of a low-relief, high elevation so-called Nevadaplano into region of similar to lower mean elevation but with significantly higher-relief.

This proposal addresses a fundamental problem in paleoclimate analysis ? the cause for rapid climatic shifts. It has been proposed that with increased global warming the Earth may undergo rapid reorganization of climate regimes once critical thresholds are reached. Identifying these rapid climate changes during times when the Earth was significantly warmer and had higher concentrations of carbon dioxide is essential for our understanding of how the Earth?s climate behaves during warming episodes. The research team has identified areas in the American West through stable isotope analysis that record rapid climatic shifts when the Earth was significantly warmer (50 to 40 million years ago). What causes these climatic shifts is unknown, however. By combining global climate models with isotope paleo-precipitation measurements it is possible to assess what may have caused these rapid climate shifts. Specifically, the project will test whether they represent regional responses to the rise of mountains or large-scale reorganization of climate.