STRATIGRAPHIC RECORD OF OCEANIC PLATEAU COLLISION AND FOREARC INITIATION IN CENTRAL AND NORTHERN ECUADOR

Subduction in northwestern South America is modulated by the presence of an accreted block, generally considered to be part of the Caribbean Oceanic Plateau, which forms the basement of the current forearc. This forearc is subaerial due to uplift related to the subduction of the Carnegie Ridge, providing a unique opportunity to study the geologic record of terrane accretion and forearc initiation. In this study we use a combination of field mapping, measurement of stratigraphic sections and detrital ages of rocks exposed along the Western Cordillera and Coastal Cordillera of Ecuador to constrain the post-collision record (<75 Ma) of deposition. Furthermore, we analyze a network of seismic industry lines tied to well data to decipher the tectonic setting of these depositional systems. We find that a fluvial depositional system is established in the Paleogene, containing abundant basaltic clasts interpreted to be sourced in the Western Cordillera, which previous studies have shown to have initiated exhumation upon collision of the plateau at ~ 75 Ma. This implies that the fluvial system would have crossed the entire forearc system. After this, a carbonate platform was established with coeval deposition in the western and eastern margins of the forearc (the Western and Coastal Cordilleras respectively). This carbonate section is overlain by a marine sequence, all of which indicate a progressive deepening of the basin likely related to regional subsidence. The interior of the forearc however presents a different setting, with large half-grabens developed shortly after accretion that accumulate several km of sediments. These features are the dominant tectonic features with evidence of only minor fault inversion, suggesting that the accreted plateau has been under extension since its inception. This is inconsistent with recently published numerical models that indicate that accreted oceanic plateaus undergo intense shortening during and after accretion. Geologic observations of this type are essential to better calibrate models of accretion and understand this important orogenic process.