DETERMINATION OF LONG-TERM STEADY STATE IN CONVERGENT OROGENS USING DETRITAL THERMOCHRONOLOGY

Detrital thermochronology provides a method for evaluating the steadiness of mountain building in a convergent orogen. We use zircon fission-track (FT) grain-age distributions from synorogenic sandstones to determine the distribution of cooling ages in the orogenic source region for the sediment. We are specifically interested in the lag time, defined as the duration between closure of the zircon FT system (~240 C) and deposition. In orogens that lack synorogenic volcanism (e.g. Alps, Himalaya), lag time is a proxy for the long-term exhumation rate, as averaged over the exhumation path of the dated zircon from FT closure at ~6 km depth and to exposure at the surface.

The evolution of the lag-time distribution for an orogen can be assembled by dating a stratigraphic-coordinated suite of synorogenic sandstone samples. The prediction is that the early "constructional phase" of a convergent orogen will be marked by a shift to shorter lag times, the post-orogenic "decay phase" by a shift to longer lag times; and steady state by the maintenance of relatively constant lag times. We refer to this last case as "exhumational steady state", because it implies that the surface of the orogen was able to maintain a relatively steady distribution of bedrock cooling ages. This condition implies a flux steady state, where the accretionary influx and erosional outflux are balanced.

We present evidence from the European Alps (Bernet et al., 2001) and Himalaya (Cerveny et al., 1988) that demonstrate that these mountain belts have been in an exhumational steady state since at least 15 Ma. Examples from modern drainages in the Alps are used to examine the influence of sampling "noise" on our interpretation. In particular, we have found that a source region might have a steady lag time but an unsteady variation in zircon yield. This result reflects the fact that lag time measures the long-term exhumation rate, whereas zircon yield is influenced by short-term variations in erosion rates.