Origin of the calderas and evolution of Roccamonfina volcano (Roman Region, Italy)

Abstract

The Roccamonfina volcano consists of a volcanic–fluvial succession dated 549 ka BP (Paleoauruncus) overlain by a 546 ka BP stratacone (Main Cone) formed by leucite-bearing lava and tephra. The main cone was dissected by four nested calderas. A ring fracture concentric with, and outside of, the central caldera is also present. Detailed mapping of the volcano, inspection of Landsat satellite images, and radiometric dating establish that the Gli Stagli and northern calderas subsided between, respectively, 546–474 and 403–374 ka BP, and that the summit part collapsed between >546 and 446 ka BP to form the central caldera. A chronologically intermediate state, dated in the range 376–323 ka BP, was identified between the 549–374 ka BP leucite-bearing High-K Series (HKS) Stage I, and the 317–96 ka BP leucite-free, Low-K Series (LKS) Stage II. This stage is made up of concurrent, HKS-LKS rocks (Stage ‘ICS’). The central, Gli Stagli, and northern calderas are older than BLT and WTT, as supported by the strong discrepancies between calculated volumes of these pyroclastic rocks and the volume of the caldera. Also, caldera collapses due to sector sliding are improbable as tested by the absence of rock-slide avalanche deposits. The collapses occurred incrementally through repetitive draining of the magma chamber(s) resulting from a succession of moderate-sized volcanic events. Gli Stagli caldera was infilled by a lake early after collapse, so that sediments alternated with leucite-tephrite lava flows and tephra within the lake. The sequence was overlain by a thick succession of young pyroclastic rocks. Downfaulting (±downsagging) cut the flat-lying Gli Stagli-filling sequence, resulting in the old Valle caldera. Successive pyroclastic and debris flows, derived by updoming of the central caldera deposits, infilled the last structure, and downfaulted part of the Valle fill producing the young Valle caldera.