Latest Eocene-Early Oligocene Magmatic and Structural Reorganization of the Sandino Forearc Basin and Central American Volcanic Arc, Nicaragua

Abstract:

The Sandino Forearc Basin (SFB) and Central American volcanic arc (CAVA) is a late Cretaceous-Recent forearc basin-volcanic arc pair located on the Pacific margin of Central America. During the Eocene-early Oligocene, this subduction system underwent a widespread magmatic and structural reorganization that is recorded by: 1) a hiatus in the CAVA record from the early Eocene (50 Ma) to the early Oligocene (30 Ma) 2) a change in the magmatic compositions from “wetter” calc-alkaline compositions in the Cretaceous-Eocene to transitional magmas in the overlying Oligocene-Miocene arc; and 3) a major folding, uplift and erosional event that affected the Sandino forearc basin during the latest Eocene and early Oligocene. This study uses a grid of 77 time-migrated seismic lines, onshore and offshore industry wells, and seismic facies and structural mapping techniques to better understand the origin of the 200-km-long, fold-thrust belt present in the deepest area of the SFB . From onlapping strata and area-depth strain analysis, these fault-propagation folds verge trenchward and display steady growth from the Oligocene to the present day. From north to south, folds amplitudes steadily increase with basement exhumation on the Nicoya peninsula that forms the southern end of the offshore Sandino forearc basin. Between the Nicoya and Burica peninsulas of southern Costa Rica, the Sandino basin is present as a highly compressed, 3-km-thick Eocene-Pliocene forearc basin (Terraba belt) that has been shortened between the Cocos Ridge and the CAVA. I discuss the structural history using two end-member models: 1) the Sandino folding event is older and unrelated to the 5 Ma Cocos event; and 2) the Sandino folding event is an edge effect of the Cocos event and records an initial phase of its collision that is not recognizable in the Terraba belt.