DILUTION OF DETRITAL ZIRCON POPULATIONS AS A QUANTITATIVE MEASURE OF FLUVIAL TRANSPORT DISTANCE AND DISCRIMINATOR OF SEDIMENTARY VERSUS TECTONIC TRANSPORT ACROSS THE DEATH VALLEY 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 stream length (Hack’s Law) can be convolved with detrital zircon age analysis of sediment to quantitatively assess fluvial transport distance of that sediment from its source. Such analyses are particularly germane to studying the fluvial transport distances of syn-tectonic 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 by an example from the Death Valley extended terrane in the central Basin and Range. Middle Miocene clastic sedimentary rocks located east of Death Valley contain a clast assemblage that includes a distinct Early Jurassic leucomonzogabbro batholith, the nearest outcrops of which lie 80 km to the WNW. Interpretation of these strata as an alluvial fan sequence required restoration of the section to a depositional position within 20 km of the source batholith, and corroborated previous restorations of extension across central Death Valley based on isopachs in Paleozoic strata and alignment of pre-extensional contractional structures. Recent re-interpretation of these strata as fluvial in origin permits significantly greater sedimentary transport distances than did the alluvial fan interpretation, and thus undercuts the convergence of multiple lines of evidence for large-magnitude extension across central Death Valley. Detrital zircon U-Pb geochronology reveals that the middle Miocene strata contain >75% Early Jurassic detrital zircons, and, given the modern areal extent of the Early Jurassic batholith, that fluvial transport of these sediments could not have exceeded 25 km, and was most likely less than 10 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.