Single‐Crystal Dating and the Detrital Record of Orogenesis

Abstract

Single-crystal dating of detrital mineral grains confers a remarkable ability to reconstruct cooling histories of orogens and to place limits on the timing, magnitude, and spatial variations of erosion. Numerous grains from a detrital sample are typically dated, and the statistical variability between populations of ages in different samples provides keys to variations in cooling histories and exhumation rates within the hinterland. Given that detrital samples comprise minerals drawn from an entire catchment, they offer an integrated perspective that is almost always unattainable with bedrock samples. Moreover, because detrital ages are preserved within stratigraphic successions, the evolution of populations of cooling ages through time and across an orogen can be reconstructed from the sedimentary record. When combined with a known hinterland 'stratigraphy' of bedrock cooling ages, studies of detrital ages in modern river systems demonstrate the fidelity of the detrital signal, and reveal both the power and limitations of detrital single-crystal dating in sedimentary basins. Low-temperature thermochronometers can be sensitive to variations in hinterland erosion of as little as 1-2 km. Although recognized previously from a theoretical viewpoint, the impact exerted on modern detrital ages by the interplay between erosion rates and lithology within tributary catchments has only recently been documented and provides a basis for refining orogenic histories using detrital ages. Documentation of the downstream evolution of detrital ages emphasizes that the distribution of ages that reaches the mouth of a river may bear little resemblance to age distributions in the headwaters. Similarly, because lithological concentrations of minerals used for singlecrystal dating can vary by many fold within the hinterland, rapidly eroding tributary catchments do not necessarily dominate populations of detrital ages. An ability to exploit detrital ages to place limits on kinematic rates within collisional orogens as a function of cooling rates provides a potent new analytical tool. If uncertainties regarding kinematic geometries, related particle pathways through orogens and steady-state assumptions can be reduced, detrital ages in both modern rivers and the recent stratigraphical record can serve to reconstruct rates of deformation and erosion and to test the viability of proposed models of orogenic evolution.

Changing exhumation patterns during Cenozoic growth and glaciation of the Alaska Range: Insights from detrital thermochronology and geochronology

“…Detrital mineral assemblages in rivers are influenced by lithologic, geomorphic, and geologic factors in the source region [ Burbank et al , ]. First, concentration of target minerals can vary widely with rock type, as we have demonstrated in our study area (Figure ).…”

Rapid exhumation of the eastern Himalayan syntaxis since the late Miocene

“…For example, an up-section lag time decrease (i.e., a decrease in lag time with decreasing depositional age) may indicate acceleration of source rock exhumation during a constructive phase of orogenesis (Bernet and as doi:10.1130/B31419.1 Geological Society of America Bulletin, published online on 27 April 2016 Garver, 2005). Constant lag time may indicate constant or steady-state exhumation (e.g., Willett and Brandon, 2002;Burbank et al, 2007).…”

Thermal and kinematic modeling of bedrock and detrital cooling ages in the central Himalaya

“…Brewer et al [2006] argued that comminution of muscovite through the Marsyandi River system had an undetectable impact, given the uncertainty in the analysis of 40 Ar/ 39 Ar ages. Moreover, the distribution of detrital zircon fission track ages from the same river [ Burbank et al , 2006] mimics that of the muscovite 40 Ar/ 39 Ar ages, but is offset toward younger ages by 1–2 Myr: an offset commensurate with the lower closure temperature of zircon.…”