Detrital zircon age distributions as a discriminator of tectonic versus fluvial transport: An example from the Death Valley, USA, extended terrane

U-Pb geochronology of detrital zircons has proven useful in identifying potential source terranes of siliciclastic sediments through the identification of distinct age peaks that can often be linked to geographically restricted regions. In cases where a sedimentary source region is geographically distinct enough to be treated as a point source, simple empirical and theoretical relationships between fluvial catchment area and downstream river length (Hack's law) can be convolved with detrital zircon age analysis of fluvial sediment to quantitatively assess the fluvial trans port distance of that sediment from its source. Such an approach is particularly germane to studying the fluvial transport distances of syntectonic sediments for which the present-day distance between sediment and source may be a function of both sedimentary and tectonic transport. This technique is illustrated with an example from the Death Valley extended terrane in the central Basin and Range (United States). Middle Miocene clastic sedimentary rocks located east of Death Valley contain a clast assemblage that includes a distinct Early Jurassic leucomonzo gabbro, the nearest outcrops of which are 80 km to the west-northwest. Previous interpretation of these strata as an alluvial fan sequence required restoration of the clastic rocks to a depositional position within 20 km of the source batholith, and corroborated restorations of extension across central Death Valley based on isopachs in Paleozoic strata and alignment of preextensional contractional structures. Recent reinterpretation of these strata as fluvial in origin permits significantly greater sedimentary transport distances than the alluvial fan interpretation, and thus undercuts the convergence of multiple lines of evidence for large-magnitude extension across central Death Valley. I demonstrate that the Middle Miocene strata contain >75% Early Jurassic detrital zircons. Given the modern areal extent of the Early Jurassic batholith from which these zircons were derived, fluvial transport of these sediments could not have exceeded 25 km, and was most likely <12 km. This estimate is independent of any assumptions regarding depositional environment, and confirms previous interpretations that >200% extension has occurred across central Death Valley since Middle Miocene time.