DETRITAL ZIRCON AGES REVEAL POST 15 MA CONVERGENCE AND PLEISTOCENE-QUATERNARY BASEMENT UNROOFING IN THE CAUCASUS

The Greater Caucasus (GC) is a young orogen transitioning from subduction to collision, making it an ideal site to study the initiation of continental collision. Further, understanding the timing of convergence and collision of the Greater Caucasus (GC) with the Lesser Caucasus (LC) may help to resolve shortening deficits and constrain the cause of Mio-Pliocene plate reorganization in the Arabia-Eurasia collision zone. However, the history of deformation in the GC remains controversial [1, 2].

We present new detrital zircon age data from 30 samples (typical n ~ 250) including modern samples from rivers that drain specific pre-Cenozoic sedimentary packages and samples of sedimentary rocks from the Cenozoic foreland basin of the GC. Zircon sources, including the GC basement, GC sedimentary cover, and LC, have distinct age signatures. Zircon age distributions of five Miocene to present samples from the Chanis River in western Georgia reveal derivation entirely from the GC, though not directly from GC crystalline basement rocks until between 2.5 Ma and early Quaternary time. Further east, two samples from the Gombori Range indicate that zircons were derived exclusively from the LC during Miocene time (~15 Ma), but that the source switched to exclusive derivation from GC sedimentary cover by Mio-Pliocene time.

Taken together, zircon spectra from the Chanis River and Gombori Range imply that until at least 15 Ma, there existed distinct depositional domains of GC and LC derived zircons. By Mio-Pliocene time, zircons derived from GC sedimentary cover dominate all samples analyzed implying tectonic closure of an intervening basin or a dramatic increase in GC sediment supply relative to LC sources. Finally, sedimentary cover was removed from the GC to expose gneissic and igneous basement in western Georgia between 2.5 Ma and early Quaternary time.

[1] Cowgill et al. (2016) Tectonics 35 (2918–2947).

[2] Vincent et al. (2016) Tectonics 35 (2948–2962).