DETRITAL ZIRCON THERMOCHRONOLOGY AT CONVERGENT MARGINS AND COLLISION ZONES

Fission track ages of detrital zircon eroded off orogenic belts retain a detailed history of the thermochronologic evolution of that orogenic belt. We illustrate the power of this technique by two examples from continental arcs and two examples from non-volcanic collisional orogens. Detritus shed off continental arcs can provide the timing of: a) volcanic and related high-level intrusive events; b) exhumation of basement rocks to the arc; and c) erosion of older volcanic rocks in the arc. Samples from well-dated stratigraphic sequences flanking continental arcs show that the transport time of volcanic detritus is nearly instantaneous, and the youngest population represents the record of syn-contemporaneous volcanism which can by used to constrain depositional age. Older grain ages in suites of FT ages from continental arcs may be related to exhumation of basement rocks, or older volcanic rocks. Combining single grain FT ages (cooling) and single grain U/Pb ages (crystallization) from the same sample allows us to determine if age populations belong to exhumed rock (FT < U/Pb) or volcanic sequences (FT=U/Pb). We illustrate these types of patterns from strata and modern sediment deposited adjacent to the Cascade arc in western NAM (US and Canada), and the Okhotsk volcanic arc built along the NE Asian Margin (Russia). In these examples, we show the utility of using the young population as a "FT depositional age", and we show that exhumation rates of basement terranes can be quantified. Detritus shed off non-magmatic collisional orogenic belts can record the full exhumation history provided samples are taken from strata that span the entire orogenic event. In these settings, detrital FT thermochronology provides the timing of: a) the removal of the pre-orogenic cover sequence; b) exhumation rates of reset rocks; and c) input of recycled orogenic detritus from flanking sedimentary sequences.