Exhumation history of orogenic highlands determined by detrital fission-track thermochronology

Abstract

A relatively new field in provenance analysis is detrital fission-track thermochronology which utilizes grain ages from sediment shed off an orogen to elucidate its exhumational history. Four examples highlight the approach and usefulness of the technique. (1) Fission-track grain age (FTGA) distribution of apatite from modern sediment of the Bergell region of the Italian Alps corresponds to ages obtained from bedrock studies. Two distinct peak-age populations at 14.8 Ma and 19.8 Ma give calculated erosion rates identical to in situ bedrock. (2) Zircon FTGA distribution from the modern Indus River in Pakistan is used to estimate the mean erosion rate for the Indus River drainage basin to be about 560 m Ma −1 , but locally it is in excess of 1000 m Ma −1 . (3) FTGA distribution of detrital apatite and zircon from the Tofino basin records exhumation of the Coast Mountains in the Canadian Cordillera. Comparison of detrital zircon and apatite FT ages gives exhumation rates of c. 200 m Ma −1 during the interval between c. 34 and 54 Ma, but higher rates ( c. 1500 m Ma −1 ) at c. 56 Ma. (4) FTGA analysis of apatite grain ages from a young basin flanking Fiordland in New Zealand indicates that removal of cover strata was followed by profound exhumation at c. 30 Ma, which corresponds to plate reorganization at this time. Exhumation rates at the onset of exhumation were c. 2000–5000 m Ma −1 . These studies outline the technique of detrital FTGA applied to exhumation studies and highlight practical considerations: (1) well-dated, stratigraphically coordinated suites of samples that span the exhumation event provide the best long-term record; (2) strata from the basin perimeter are the most likely to retain unreset detrital ages; (3) the removal of ‘cover rocks’ precedes exhumation of deeply buried rocks, which retain a thermal signal of the exhumation event; (4) steady-state exhumation produces peak ages that progressively young with time and have a constant lag time; (5) same-sample comparison of zircon and apatite peak ages is best in sequences with high-uranium apatite grains (>50 ppm), and peak-ages statistics can be improved by counting numerous apatite grains (>100).

Fundamentals of detrital zircon fission-track analysis for provenance and exhumation studies with examples from the European Alps

“…One of the real promises of the FTGA technique is its ability to address the long-term exhumation of orogenic belts, because sediments in a stratigraphic sequence capture a representative picture of source exhumation through time (Cerveny et al, 1988;Brandon and Fundamentals of detrital zircon fission-track analysis Vance, 1992;Garver and Brandon, 1994b;Lonergan and Johnson, 1998;Carter and Moss, 1999;Garver et al, 1999;Carter and Bristow, 2000;Spiegel et al, 2000;Bernet et al, 2001). At this point, we can be reasonable assured that the basin strata captures representative samples of the orogenic belt through time, and there is a reasonably quantitative transfer of material.…”

“…We followed the approach of Galbraith and Green (1990) in using their binomial peakfit method (Brandon, 1996(Brandon, , 2002. Zircon FT peak ages are a proxy for long-term exhumation rates, where cooling occurs by erosion or normal faulting, and not following magmatic events (Cerveny et al, 1988;Garver et al, 1999). The Alps are basically free of recent volcanism.…”

“…Fission-track (FT) analysis of detrital zircon has become an important tool for the study of sediment provenance and long-term exhumation of orogenic mountain belts (Hurford et al, 1984;Zeitler et al, 1986;Cerveny et al, 1988;Hurford andCarter, 1991, Brandon andVance, 1992;Garver and Brandon, 1994a and b;Lonergan and Johnson, 1998;Carter, 1999;Garver et al, 1999;Spiegel et al, 2000;Bernet et al, 2001;Garver and Kamp, 2002). In this paper we examine the ability of fission-track grain-age (FTGA) distributions for detrital zircon from modern river sediments to resolve bedrock cooling ages in an orogenic source region, and consider the procedures used in this kind of study.…”

“…In active orogenic settings, zircons cool in the crust by tectonic and erosional exhumation or by conductive cooling following volcanism or shallow plutonism. Therefore, in geologic settings with little or no igneous activity, zircon FT ages can be used as a proxy for long-term exhumation (Cerveny et al, 1988;Garver et al, 1999). Many orogens have deeply exhumed metamorphic internal zones where the FT ages for zircon provide information about post metamorphic cooling and exhumation.…”

Thermochronology of the Torlesse accretionary complex, Wellington region, New Zealand

“…Detrital FT thermochronology differs from conventional FT thermochronology in that the distribution of FT grain ages within a sample, particularly for sediments, can arise from crystals originating from a variety of thermotectonic source terrains, unlike a conventional FT age, where all the grains dated from a sample have had a common thermal history. Hence, in detrital FT studies one explores the provenance and age information contained in rocks that have not been partially overprinted since deposition [e.g., Hurford et al, 1984;Garver and Brandon, 1994;Garver et al, 1999]. Both conventional and detrital FT thermochronological approaches have been applied in this study, thereby establishing maximum palcotemperatures in the Rakaia Terrane and in blocks of sandstone in the Rimutaka Melange and constraining depositional ages in the accretionary complex farther east.…”