Large-scale catastrophic flank collapses in a steep volcanic ridge: The Pico-Faial Ridge, Azores Triple Junction

Large-scale flank collapses are common in the geological evolution of volcanic ocean islands in the Atlantic. To date, catastrophic lateral collapses in the Azores Islands have been difficult to identify, leading to suggestions that a lack of events may relate to the relatively small size of the islands. Here we show evidence for two major collapses on the northern flank of Pico Island (Pico-Faial volcanic ridge, central Azores), suggesting that this island had a collapse incidence similar to that of other Atlantic volcanic islands. The study is based on the analysis of: (1) offshore and onshore high-resolution digital elevation models; (2) field data focused on the N flank; and (3) new K-Ar ages on selected lava flow samples. Pico sub-aerial northern flank is marked by two conspicuous arcuate shaped depressions concave towards the sea, here interpreted as landslide scars. A main debris field is observed offshore the largest depression. This deposit has 20 km of maximum length, covers ca. 150 km(2), is composed of meter to hectometer blocks, and has an exposed volume here estimated between 4 and 10 km(3), though the actual volume probably exceeds 10 km3. Debris flow towards the ESE was apparently determined by the slope of the narrow WNW-ESE S. Jorge channel. Young lava flows cascade over the interpreted scars, thus concealing the older volcanic sequence(s) affected by the landslide(s). New K-Ar ages measured on these lava flows provide a minimum age of ca. 70 ka for the largescale collapse(s) in Pica's northern flank. (C) 2014 Elsevier B.V. All rights reserved.