Locally Thin Crust and High Crustal V_P/V_S Ratio Beneath the Armenian Volcanic Highland of the Lesser Caucasus: A Case for Recent Delamination

Abstract

The Arabia-Eurasia continental collision created the Caucasus Mountains and the Anatolian and Iranian plateaus. Between the two plateaus, the Armenian Highland features young Holocene-aged volcanoes. In this study, the P-wave receiver functions from a new seismic array reveal a thick crust (up tõ 52 km thick) beneath the Central Greater Caucasus and an unusually thin crust (32-35 km thick) beneath the northwestern part of Armenia near the Aragats stratovolcano and Gegham volcanic ridge formed by Pleistocene to Holocene monogenetic cinder cones. The average crustal V P /V S ratio in the Armenian Highland is anomalously high (≥1.9), with the highest value approaching 2.1 under the Gegham Ridge. Such high V P /V S ratios cannot be explained by an overall mafic crustal composition. Instead, the presence of partial melts is inferred in the lower crust based on the depth of the low-velocity structure obtained by inversion of receiver function waveforms. Our study suggests that the postcollisional volcanism was potentially facilitated by the small-scale removal of lithospheric mantle, resulting in a localized thinner crust balanced by thermal buoyancy and dynamic flow in the uppermost mantle beneath the Armenian Highland.

Pervasive Crustal Volcanic Mush in the Highly Stretched Sunda Plate Margin of Northern Sumatra

“…The estimated V p / V s ratio varies from 1.65 to 2.19 (±0.04–0.11; ±0.09 on average), with an average of ∼1.98 (Figure 3b). The average V p / V s ratio is much higher than the global continental average of ∼1.77 (Christensen, 1996), but instead is closer to the V p / V s ratios in active volcanic regions (1.87 and above) (Ji et al., 2009), with values higher than 1.95 reported in many volcanic areas (e.g., Janiszewski et al., 2013; Lin et al., 2020; Rao et al., 2015), including the Sunda arc (Syuhada et al., 2016; Wölbern & Rümpker, 2016). The crustal average V p values are 4.9–6.3 km/s (±0.1–0.5 km/s; ±0.3 km/s on average), with an average of ∼5.4 km/s (Figure 3c), close to the value of ∼5.1 km/s in the Red Sea Rift volcanic zone (Reed et al., 2014) but much smaller than the global continental average of 6.45 km/s (Christensen & Mooney, 1995).…”