Kilometers-scale subsidence of the inner Puerto Rico Trench wall since the Pleistocene

We have observed a thin veneer of Pleistocene-age carbonate rocks on the southern side of Mona Block, a seamount northwest of Puerto Rico, above a water depth of 1800 m. Mona Block is located within the inner wall of the Puerto Rico Trench. A similar veneer of carbonate rocks (corals and pavement) was encountered during a deep submergence vehicle (DSV) Alvin dive in 1976 on the north side of Mona Block at depths of 3650–2900 m. We also observed a remnant Oligocene–Pliocene carbonate platform at a depth of 2080 m on the east side of the block. Restoring the carbonate outcrops to near sea level implies that Mona Block, with a summit currently at 670 m bsl, was an island with an elevation of ≥1000 m until the mid-Pleistocene and has subsided and tilted to the north thereafter. We suggest that the observed subsidence was caused by the foundering or retreating North American slab, which gave rise to the anomalously deep Puerto Rico Trench. Our estimated trench wall subsidence varies nonlinearly from ~800 m in the distal part to ≥3700 m near the trench. Carbonate platform subsidence north of Puerto Rico is older (mid-Pliocene), suggesting that subsidence and, by inference, trench collapse had propagated westward with time. Mona Block is thus a yardstick for studying mantle processes associated with slab retreat and foundering.