New Pliocene-Pleistocene 40Ar/39Ar ages fill in temporal gaps in the Nicaraguan volcanic record

摘要

The volcanic record of western Nicaragua documents a significant lull in volcanic activity that has persisted from the late Miocene (-7 Ma) to the formation of the modern volcanic front around 350 ka. This study fills this gap for the first time with samples collected in Northwest Nicaragua between Cosigiiina and San Cristobal volcanoes and with samples collected from the Nicaraguan Depression. We found two previously unknown volcanic units ranging from 3.6 to 1.3 Ma and the improved volcanic record allows us to reconstruct the geochemical evolution of the Nicaraguan arc. U/Th values increased by nearly threefold since the Miocene following the "carbonate crash" at 10 Ma, when dominantly carbonate sediment deposition shifted toward hemipelagic sediment deposition. This transition was thought to be abrupt, however our new data show that it took place gradually over the last 7 Ma. Northwest Nicaragua is a particularly interesting case study because it contains Middle Miocene volcanism on either side of the Nicaraguan Depression, the Coyol Formation (25-7 Ma) to the East and the Tamarindo Formation (14.7-11.7 Ma) to the West. The presence of Mid Miocene volcanism on either side of the Nicaraguan Depression has led to the hypothesis that the two coeval units, currently separated by -100 km, were once connected and have since been separated by extension. Here, we present data suggesting that the Tamarindo and Coyol are geochemically distinct and therefore cannot be considered part of the same unit.