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

Abstract

Extension within a continental back‐arc basin initiates within continental rather than oceanic lithosphere, and the geochemical characteristics of magmatic rocks within continental back‐arcs are poorly understood relative to their intraoceanic counterparts. Here, we compile published geochemical data from five exemplar modern continental back‐arc basins—the Okinawa Trough, Bransfield Strait, Tyrrhenian Sea, Patagonia plateau, and Aegean Sea/Western Anatolia—to establish a geochemical framework for continental back‐arc magmatism. This analysis shows that continental back‐arcs yield geochemical signatures more similar to arc magmatism than intraoceanic back‐arcs do. We apply this framework to published data for Triassic‐Jurassic magmatic rocks from the Caucasus arc system, which includes a relict continental back‐arc, the Caucasus Basin, that opened during the Jurassic and for which the causal mechanism of formation remains debated. Our analysis of 40Ar/39Ar and U‐Pb ages indicates Permian‐Triassic arc magmatism from ∼260 to 220 Ma due to subduction beneath the Greater Caucasus and Scythian Platform. Late Triassic (∼220–210 Ma) collision of the Iranian block with Laurasia likely induced trench retreat in the Caucasus region and led to migration of the Caucasus arc and opening of the Caucasus Basin. This activity was followed by Jurassic arc magmatism in the Lesser Caucasus from ∼180 to 140 Ma and back‐arc spreading in the Caucasus Basin from ∼180 to 160 Ma. Trace element and Sr‐Nd isotopic data for magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental back‐arcs and that the source to the Lesser Caucasus arc became more enriched at ∼160 Ma, likely from the cessation of back‐arc spreading.

Episodic evolution of a protracted convergent margin revealed by detrital zircon geochronology in the Greater Caucasus

“…This proposed distinct phase of Permian to Triassic deposition is consistent with the stratigraphy of the Northern Caucasus, where Tikhomirov et al (2004) described intensely folded Devonian to Carboniferous strata unconformably overlain by Triassic siliciclastic, carbonate and volcanic strata. The volcanic component of these Triassic rocks has been interpreted as originating in a Permian to Triassic continental‐margin arc that formed north of the present‐day Greater Caucasus above a north‐dipping (present orientation) subduction zone (Figures 2d and 7; Natal'in & Şengör, 2005;Okay & Nikishin, 2015 ; Vasey et al, 2021). We infer that Triassic deposition of samples SV2 and SV3 in the present‐day Greater Caucasus, south of this arc, took place in a forearc basin and interpret the Permian to Triassic strata identified in this study as a more distal part of this forearc basin, rather than an early phase of deposition in the back‐arc basin that subsequently opened in Jurassic time (Figure 7).…”

“…Gamkrelidze & Kakhazdze, 1959; Gudjabidze, 2003; Kandelaki & Kakhazdze, 1957; Potapenko, 1964) and thus provide insight into the major phase of back‐arc basin evolution in the Caucasus Basin (e.g. Cowgill et al, 2016; Vasey et al, 2021; Vincent et al, 2016; Zonenshain & Le Pichon, 1986).…”

“…By Permian to Early Triassic time, northward subduction had initiated along the Laurasian margin beneath the Greater Caucasus, leading to arc magmatism within the present‐day Scythian Platform and Northern Caucasus (Figure 2d; Alexandre et al, 2004; Gerasimov et al, 2015; Natal'in & Şengör, 2005; Okay & Nikishin, 2015; Tikhomirov et al, 2004; Vasey et al, 2021). The younger sedimentary cover of the Scythian Platform obfuscates the geological record of this phase, and only a few Triassic arc rocks have been directly dated in well cores from the Scythian Platform or outcrop samples in the Northern Caucasus (Alexandre et al, 2004; Gerasimov et al, 2015).…”

Diverse Deformation Mechanisms and Lithologic Controls in an Active Orogenic Wedge: Structural Geology and Thermochronometry of the Eastern Greater Caucasus

“…The Caucasus region has a complex tectonic history, with multiple episodes of subduction, terrane accretion, and rifting during Phanerozoic time (Adamia et al., 2011; Şengör, 1984; Stampfli, 2013; Vasey et al., 2020). The successions of marine sedimentary rocks and volcaniclastic rocks exposed across the majority of the orogen record Jurassic to Eocene deposition in a backarc basin, termed the Greater Caucasus Basin, on the Eurasian margin north of the Jurassic to Eocene Lesser Caucasus magmatic arc (Figure 2; Cowgill et al., 2016; Nalivkin, 1976; Tye et al., 2021; Vasey et al., 2021; Vincent et al., 2016; Zonenshain & Pichon, 1986). The Greater Caucasus Basin was subsequently closed during the Cenozoic Arabia‐Eurasia collision (Adamia et al., 2011; Cowgill et al., 2016; Khain, 1975; Vincent et al., 2007).…”

Crystals Detail Magma Production and Crust Formation at an Active Back-Arc Submarine Volcano

“…1). The Tyrrhenian represents an example of the seafloor spreading stage in a continental back-arc setting 4 , formed following continental crust rifting behind the Aeolian Volcanic Arc and Calabrian suture zone during subduction and retreat of the Ionian plate 5 . Magmatic activity of the MV took place in the last 0.7 million years, producing primarily basaltic and basaltic andesite lavas; volumetrically minor andesite lavas were recovered at the summit cone of the volcano 6 (Fig.…”