Detrital zircon geochronology and processes in accretionary wedges

The detrital zircon U-Pb geochronology has been widely employed to analyze accretionary wedges over the past decade. The goal of this paper is, first, to briefly review this rapidly growing field of research and then to explore the exciting potential of the detrital zircon geochronology for interpreting processes in accretionary wedges. A few general conclusions may be derived especially from examples along the Circum-Pacific Belt as well as from the Blovice accretionary complex, Bohemian Massif, examined here in detail. (1) The detrital zircon age spectra are typically a mixture of cratonic and arc-derived material. While the former helps to resolve the provenance of various terranes, the latter may detect changing magmatic arc tempos. (2) The maximum depositional ages, inferred from the youngest zircon ages, are close to the true ones as the trench-fill sediments are sourced from active arcs through rapid erosion and transport to the wedge toe. (3) The maximum depositional ages of the oldest unit within an accretionary wedge provide a key information on the onset of accretion, which, however, may be delayed from the subduction initiation. The onset of strongly accretionary behavior may be linked to changes in the subduction parameters and arc activity. Furthermore, we introduce a new concept of the maximum depositional Age-Distance Curves (ADCs). These curves are plots of the maximum depositional ages on the abscissa with respect to a distance along a sampling transect (running across-strike of different tectonostratigraphic units composing the accretionary wedge) on the ordinate and may reveal a wealth of information on a variety of dynamic processes within accretionary wedges. Four main types of the ADCs are defined on the basis of their overall shape and course: (1) Linear ADCs may track continuous accretion whereas (2) variably sloping ADCs may reflect tectonic erosion, increase in the rate of deposition and accretion, or tectonic duplication. (3) Stepped ADCs may record the latter two processes, but also out-of-sequence thrusting while (4) sawtooth ADCs may result from emplacement of older units within younger (olistoliths) or, vice versa, from tectonic underplating and exhumation. It should be noted, however, that the ADCs are nonunique in some cases, i.e., the same curves may reflect different processes. Finally, we point out several issues that still remain poorly understood and would deserve attention in future research. For instance, detailed sampling along transects is desirable to quantify temporal variations in the rates of sediment-supply to the trench and in the rates of accretion. It is also not clear whether the maxima and minima in the zircon age spectra truly match the changing arc tempos, or whether they are affected by other factors such as erosion or sediment transport. One of the exciting fields is detrital zircon U-Pb and fission-track multichronology, which may elucidate the complete sediment trajectories. And, last but not least, the detrital zircon (multi-)chronology should be integrated with detailed sedimentological and structural studies to address the key issue: how the accretionary wedges respond to the temporally evolving subduction zone dynamics.