Sedimentary response to a collision orogeny recorded in detrital zircon provenance of Greater Caucasus foreland basin sediments

Abstract

The Greater Caucasus orogen on the southern margin of Eurasia is hypothesized to be a young collisional system and may present an opportunity to probe the structural, sedimentary and geodynamic effects of continental collision. We present detrital zircon U-Pb age data from the Caucasus region that constrain changes in sediment routing and source exposure during the late Cenozoic convergence and collision between the Greater Caucasus orogen and the Lesser Caucasus, an arc terrane on the lower plate of the system. During Oligocene to Middle Miocene time, following the initiation of deformation within the Greater Caucasus, marine sandstones and shales were deposited between the Greater and Lesser Caucasus, and detrital zircon age data suggest no mixing of Greater Caucasus and Lesser Caucasus detritus. During Middle to Late Miocene time, Greater Caucasus detritus was deposited onto the Lesser Caucasus basin margin, and terrestrial, largely conglomeratic, sedimentation began between the Greater and Lesser Caucasus. Around 5.3 Ma, upper plate exhumation rates increased and shortening migrated to pro-and retro-wedge fold-thrust belts, coinciding with the initiation of foreland basin erosion. Sediment composition, provenance and structural data from the orogen together suggest the existence of a wide (230-280 km) marine basin that was progressively closed during Oligocene to Late Miocene time, probably by subduction/lithospheric underthrusting beneath the Greater Caucasus, followed by initiation of collision between the Lesser Caucasus arc terrane and the Greater Caucasus in Late Miocene to Pliocene time. The pace of the transition from hypothesized subduction to collision in the Caucasus is consistent with predictions from numerical modeling for a system with moderate convergence rates (<13 mm/yr) and hot lower plate continental lithosphere. Basement crystallization histories implied by our detrital zircon age data suggest the presence of two pre-Jurassic sutures between stable Eurasia and the Lesser Caucasus, which likely guided later deformation.

A Preliminary Framework for Magmatism in Modern Continental Back‐Arc Basins and Its Application to the Triassic‐Jurassic Tectonic Evolution of the Caucasus

“…Positive εNd values and 87 Sr/ 86 Sr values less than 0.7045 suggest that the Nb‐Ta anomalies are dominantly derived from the mantle source, rather than contamination by continental crust (Figure 13c; e.g., Ernst et al., 2005). The major episode of Jurassic arc magmatism in the Lesser Caucasus occurred between ∼180 and 140 Ma, as suggested by the numerous and continuous zircon ages spanning this age range in the ∼170 Ma detrital zircon peaks (Figure 9; Cowgill et al., 2016; Tye et al., 2020).…”

“…Three Jurassic sandstone samples document peaks at ∼240 Ma spanning ∼260–220 Ma (K3 and GC‐41; Allen et al., 2006; Vasey et al., 2020) and at ∼170 Ma spanning ∼180–160 Ma (GC‐41 and NEGC; Allen et al., 2006; Cowgill et al., 2016). These ∼240 Ma and ∼170 Ma peaks are present in a number of detrital zircon analyses of modern river sediments draining the central and eastern Greater Caucasus, and individual zircon ages of ∼260–220 Ma and ∼180–160 Ma are present even in samples where they do not form discernible peaks (Cowgill et al., 2016; Tye et al., 2020). A single sample (EGC‐5) draining poorly understood Cretaceous volcaniclastic rocks of the Vandam zone yields numerous ages younger than 140 Ma and may represent arc magmatism in the Lesser Caucasus or a later episode of back‐arc magmatism in the Cretaceous (Tye et al., 2020).…”

“… Detrital zircon ages from samples with Triassic‐Jurassic ages from the Greater and Lesser Caucasus indicating times of peak magmatic activity at ∼260–220 Ma and ∼180–160 Ma (Greater Caucasus—blue bars) and ∼180–140 Ma (Lesser Caucasus—green bars), inferred from widths of common age populations in samples of both Jurassic sandstone (yellow, depositional ages indicated in italics) and modern river sediment (pink) (GC‐41 from Allen et al., 2006; NEGC, Kumuk, Tovuz from Cowgill et al., 2016; K3 from Vasey et al., 2020; EGC, CGC, and LC samples from Tye et al., 2020). Curves show kernel density estimates calculated using IsoplotR (Vermeesch, 2018) clipped to 260–140 Ma.…”

“…Detrital zircon U‐Pb analyses from modern rivers draining the Lesser Caucasus yield few Triassic ages, though a few grains date from ∼240 to 200 Ma (Figure 9; Cowgill et al., 2016; Tye et al., 2020). Much more prominent is a Middle Jurassic peak at ∼170 Ma, with continuous ages ranging from ∼180 to 140 Ma (Cowgill et al., 2016; Tye et al., 2020).…”

“…Arc magmatism resulting from subduction beneath the Lesser Caucasus had initiated by ∼180 Ma, as suggested by U‐Pb and 40 Ar/ 39 Ar data (Figures 14c and 15c). For example, modern rivers draining the Lesser Caucasus contain zircons as old as ∼180 Ma in the Middle Jurassic zircon age peaks centered at ∼170 Ma (Figure 9; Cowgill et al., 2016; Tye et al., 2020), and a metamorphic 40 Ar/ 39 Ar age of ∼182 Ma is reported from the Khrami massif in the Lesser Caucasus (Figure 12; Rolland et al., 2011). Trace element patterns in Middle Jurassic rocks from the Lesser Caucasus are consistent with petrogenesis in an island arc setting, with relatively flat REE patterns and negative Nb‐Ta anomalies, although the flat REE patterns suggest a relatively depleted mantle source (Figure 13; Galoyan et al., 2018; Mederer et al., 2013, 2014).…”