Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

The thermochronometric systems discussed in this volume broadly share three features: parent isotopes, daughter products, and one or more time-dependent, temperature-sensitive processes by which daughter products are altered or lost. If these processes can be measured in the laboratory, and their behavior confidently extrapolated to geological time scales, it becomes possible to construct a forward model of the system that predicts how a given instance of it will evolve assuming a particular starting arrangement and subsequent time-temperature history.

Once a forward model has been created and verified, it then becomes possible to apply it in the inverse sense: given a measured ending condition and an assumed starting one, find the intervening time-temperature history. In general, because of information loss, limited precision of measurements, and lack of system uniqueness, more than one history is consistent with a given ending condition. As a result, an inverse model solution usually consists of a set of thermal histories that are consistent with the measured data, as judged by some statistical criterion.

This chapter will concentrate on two low-temperature thermochronometers: fission-track (primarily for apatite) and (U-Th)/He. All of the calculations described here are implemented in a computer program called “HeFTy,” which is available with this volume (see Ehlers et al. 2005).

A theme that will be touched upon throughout this chapter is that forward and inverse models are only as good as the data and assumptions behind them. Although this principle of course holds for all scientific investigations, it is often obscured when such details are packaged in user-friendly software that produces publication-ready graphics.

Fission tracks form continuously over time at a rate dependent solely upon the concentration of uranium present. Earlier-formed fission tracks tend to be shorter than later-formed tracks, as they will have had more time to anneal, and may have experienced …

Forward and Inverse Modeling of Low-Temperature Thermochronometry Data

Origin and History of the South China Sea Basin

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 ∼10 5 –10 7 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.

/pdf/using-thermochronology-to-understand-orogenic-erosion-3sbe4s46h9.pdf

Using thermochronology to understand orogenic erosion

The possibility of dating minerals by the accumulation of ^4He from U and Th decay
has been recognized for many years (e.g., Strutt 1905), but in the century since the idea
was first conceived, the method has rarely been applied successfully. After several
investigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957;
Fanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973;
Leventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding
unreliable and usually low ages, presumably as a result of diffusive He loss possibly
associated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the
method by proposing that in the case of apatite, He ages might be meaningfully
interpreted as ages of cooling through very low temperatures. Laboratory diffusion data
presented by these authors indicated a closure temperature of about 100oC, a value
supported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al.
1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages
increase systematically with sample elevation in a mountain range, as expected for
exhumation-induced cooling through a low closure temperature. Based on the strength of
these results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997;
House et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has
focused on applications of apatite He thermochronometry. There is also renewed interest
in He dating of other U- and Th-bearing minerals both for dating mineral formation and
for thermochronometry. For example, Lippolt and coworkers have undertaken detailed
studies of He diffusion and dating of various phases, most notably hematite formed in
hydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et
al. 1993; Wernicke and Lippolt 1994a,b).
Here I present an overview of recent techniques, calibrations, and applications of the
(U-Th)/He dating method; Hurley (1954) provides an excellent summary of earlier work in
this field. Much of this paper focuses on apatite, because the He behavior and requisite
analytical techniques are better established for this phase than for other target minerals,
such as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior
required for thermochronometric applications, yet recent work is also considering applications
to direct dating, for example, of young tephras (Farley et al. 2001).

(U-Th)/He Dating: Techniques, Calibrations, and Applications

Annealing kinetics of fission tracks in zircon: an experimental study

One-hour isochronal annealing experiments were carried out on zircon crystals KT06, separated from a rhyolitic welded tuff of the Koto Rhyolites, southwest Japan, in order to study thermal annealing characteristics of spontaneous fission tracks in zircon. Variations of track density, horizontal confined track length and track angular distribution were measured at various temperatures between 400° and 750°C. Track density and track length begin to decrease around 450°C and reach zero around 750°C. Both time and anisotropy of track etching increase drastically for the samples annealed above 500°C because of the loss of accumulated radiation damage derived from α-decay of U and Th. Angular distributions of etched tracks in those samples suggest no significant apparent reduction of etching efficiency due to increased etching anisotropy, so long as adopted etching criteria are strictly maintained. Angular distribution measurements also reveal anisotropic annealing characteristics of tracks, which may produce an additional sampling bias in track length measurement.

Thermal annealing characteristics of spontaneous fission tracks in zircon

https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/186661/1/mfskugm%20053001_002_001.pdf

Susumu Nishimura

Papers

Annealing kinetics of fission tracks in zircon: an experimental study

Thermal annealing characteristics of spontaneous fission tracks in zircon

Fissipon Track Dating Using External Detector Method : a Laboratory Procedure

Confined fission-track length measurement of zircon: assessment of factors affecting the paleotemperature estimate

Towards zircon fission-track thermochronology : Reference framework for confined track length measurements