Using Thermochronology to Understand Orogenic Erosion

Erosion of orogenic mountain ranges exhumes deeply buried rocks and controls weathering, climate, and sediment production and transport at a variety of scales. Erosion also affects the topographic form and kinematics of orogens, and it may provide dynamic feedbacks between climate and tectonics by spatially focused erosion and rock uplift. Thermochronology measures the timing and rates at which rocks approach the surface and cool as a result of exhumation. Relatively well-understood noble gas and fission-track thermochronometric systems have closure temperatures ranging from ∼60 to ∼550◦C, making them sensitive to exhumation through crustal depths of about one to tens of kilometers. Thus, thermochronology can constrain erosion rates and their spatial-temporal variations on timescales of ∼105–107 years, commensurate with orogenic growth and decay cycles and possible climate-tectonic feedback response times. Useful methods for estimating erosion rates include inverting ages for erosion rates using crustal thermal models, vertical transects, and detrital approaches. Spatial-temporal patterns of thermochronometrically determined erosion rates help constrain flow of material through orogenic wedges, orogenic growth and decay cycles, paleorelief, and relationships with structural, geomorphic, or climatic features. 419 A nn u. R ev . E ar th . P la ne t. Sc i. 20 06 .3 4: 41 946 6. D ow nl oa de d fr om a rj ou rn al s. an nu al re vi ew s. or g by Y al e U ni ve rs ity S O C IA L S C IE N C E L IB R A R Y o n 05 /0 2/ 06 . F or p er so na l u se o nl y. ANRV273-EA34-14 ARI 17 April 2006 23:42 Rock uplift/surface uplift: vertical motion of rock or a portion of the Earth near or at the surface, relative to a datum, such as sea level Erosion: the surficial removal of mass at a point in the landscape by both mechanical and chemical processes Denudation: the removal of rock or soil by tectonic and/or surficial processes at a specified point at or under Earth’s surface. Denudation can be tectonic (normal faulting or ductile thinning) or erosional Exhumation: the unroofing history or path of a rock toward Earth’s surface, as a result of a denudational process. Exhumation is also either tectonic or erosional INTRODUCTION Orogens (mountain ranges formed by tectonic processes) control the patterns and scales of a wide variety of phenomena, including weathering, erosion, climate, hydrology, sediment transport and deposition, biotic distributions, and natural resources and hazards. The configuration and architecture of orogens also provide clues to the dynamics of mantle convection and the movement of tectonic plates, and the exhumed rocks within them provide samples of the deep crust and upper mantle. Understanding the growth and decay of orogens thus provides a basic framework for understanding many other natural processes and their interactions. Three interrelated processes are commonly used to describe the tectonic geomorphology of orogens: rock uplift, surface uplift, and erosion. As clarified by England & Molnar (1990), rock and surface uplift describe the vertical motion of rock or a portion of Earth’s surface, respectively, relative to a datum, such as sea level, that is suitably fixed. Erosion is the surficial removal of mass at a point in the landscape by both mechanical and chemical processes. It follows, then, that the difference between rock uplift and surface uplift is erosion. For example, surface uplift, which contributes to mountainous topography, only occurs when erosion is slower than rock uplift. Erosion is one type of the broader process of denudation, which, following Ring et al. (1999a), is the removal of rock or soil by tectonic and/or surficial processes at a specified point at or under Earth’s surface. The other types of denudation are tectonic: normal faulting and ductile thinning. Incidentally, it is important to note that crustal thickening by ductile or brittle strain (e.g., thrusting) causes burial, not denudation, although thickening can produce high topography, which may result in erosion. Another term that is frequently used in studying orogenic evolution is exhumation, which Ring et al. (1999a) defined as the unroofing history of a rock, as caused by tectonic and/or surficial processes. Thus, exhumation is the history of or path taken by a particular rock toward the surface, as a result of a denudational process. Erosion plays a critical role in orogenic evolution in several ways. First, it often follows rock and surface uplift in space and time, providing an indirect constraint on the spatial and temporal patterns of uplift in a landscape, which may be otherwise difficult to observe directly. More generally, erosion is a dynamic link between tectonic uplift and many other processes, including chemical weathering and long-term climate change, and sediment production, routing, and deposition. Most immediately, however, erosion directly influences not only topographic decay, but also growth of an orogen, by modulating the pattern and rates of surface uplift. Because erosion is also related to climate (e.g., precipitation), it provides an important feedback between climate and tectonics. Thus, interpreting how climate affects orogeny requires understanding erosion. Rates of erosion over short timescales (1–10 years) can be estimated by sediment loads carried by rivers, but this can be difficult to extrapolate to longer timescales, largely because of the influence of large, infrequent erosional events, such as landslides (Hovius et al. 1997, Kirchner et al. 2001, Burbank 2002). Cosmogenic nuclide measurements made on fluvial sediment or in-situ materials can provide erosion rate estimates over timescales of typically 103–104 years, capturing rates closer to 420 Reiners · Brandon A nn u. R ev . E ar th . P la ne t. Sc i. 20 06 .3 4: 41 946 6. D ow nl oa de d fr om a rj ou rn al s. an nu al re vi ew s. or g by Y al e U ni ve rs ity S O C IA L S C IE N C E L IB R A R Y o n 05 /0 2/ 06 . F or p er so na l u se o nl y. ANRV273-EA34-14 ARI 17 April 2006 23:42 those over longer terms, and integrating an effective erosion rate signal through several meters depth (Brown et al. 1995, Bierman & Steig 1996, Granger et al. 1996, Kirchner et al. 2001, Riebe et al. 2003). However, many important geologic processes and feedbacks operate on timescales commensurate with large-scale rock deformation, at roughly 105–107 year scales, requiring understanding of the spatial-temporal patterns of erosion on longer timescales. Erosion rates in active and decaying mountain ranges are typically 0.05–10 km Myr−1, with total erosion reaching depths of about 5–30 km. These depths are too great for cosmogenic approaches, and, in many cases, total erosion depths are too shallow to estimate using thermobarometricgeochronologic constraints. Many radioisotopic thermochronometers, however, are well suited to this task because they are sensitive to temperature changes between about 60–550◦C, corresponding to crustal depths of about 2–10 km. In this chapter, we review general aspects of the use of thermochronology for the study of orogenic erosion, beginning with the fundamental bases of thermochronometric systems commonly used for this purpose, with an emphasis on K/Ar (and 40Ar/39Ar), (U-Th)/He, and fission-track methods. We then review some basic concepts of the thermal structure of the shallow crust and how it is influenced by erosion and topography, and how these factors can be accounted for in the interpretation of thermochronologic data. We then describe several important techniques used in both active and decaying orogens for interpreting the spatial and temporal patterns of erosion, and provide examples illustrating the insights these approaches provide.