Using Detrital Geochronologic and Thermochronologic "Double-Dating" to Constrain Depositional Age, Provenance, and Exhumation Signals in Ancient Forearc Basins

Abstract:

The application of coupled detrital zircon U-Pb geochronology and (U-Th)/He thermochronology to sedimentary basins has the potential for unprecedented details about grain provenance, depositional age and source and basin exhumation signals. Although several studies have implored this technique, it is underutilized and may prove useful in geologic settings that are traditionally difficult to explore. For example, constraining the depositional age of strata in ancient forearc basins is challenging as many horizons are devoid of fossils and post-burial diagenesis of limestone beds limits biostratigraphic age control. This study applies U-Pb detrital zircon geochronology to clastic rocks from the Cretaceous-Eocene Xigaze forearc basin in southern Tibet to (1) to determine the provenance of forearc basin strata and (2) to constrain a maximum depositional age of stratigraphic horizons using the youngest distinct age group from a sample. In addition, (U-Th)/He thermochronology was applied to a subset of the detrital zircons on which U-Pb ages were previously determined in order to determine the timing of exhumation of Xigaze forearc strata and its source region.

The use of young populations of zircons is a good method for age control in the Xigaze forearc basin because magmatism in the source area was more-or-less continuous and the lag time between the youngest zircons in a sample and the time of that samples deposition is likely relatively small. A total of 2,330 zircon grains yielded ages with acceptable precision and concordance for geochronologic interpretation. Together with sandstone petrography, the detrital zircons indicate that the primary source of detritus in the basin from ~113 to 54 Ma was the Gangdese magmatic arc. Analysis of the youngest age component of individual samples reveals a decrease in the youngest ages upsection, consistent with maximum depositional ages that are close to the likely true depositional age based on intervening tuff layers. Double-dated zircon grains from all stratigraphic levels reveal that the basin was buried to > 200°C prior to 26 Ma as all zircons are significantly younger than the depositional age of their stratigraphic horizons. Basin exhumation is constratined to be between 30-17 Ma.