From accretion to forearc basin initiation: The case of SW Ecuador, Northern Andes

Abstract

The SW of Ecuador offers a great opportunity to study the long-term behavior of an almost entire forearc system, from the external accretionary prism to the landward limit of the forearc basin. A combination of field observations, LA-ICP-MS U-Pb dating on zircon and interpretation of different vintages of unpublished industrial seismic records are used to study the evolution of the forearc system of SW Ecuador including the accretionary prism and the forearc depocenter. The youngest dates, obtained from U-Pb dating on zircon grains believed to be derived from the arc, define the best estimate for the age of sedimentation and permits a clear description of the temporal and spatial evolution of the accretionary, post accretionary and forearc basin series between 60 Ma and 10 Ma. This reinforces the idea that forearc sediments

Cenozoic Stages of Forearc Evolution Following the Accretion of a Sliver From the Late Cretaceous‐Caribbean Large Igneous Province: SW Ecuador‐NW Peru

“…Following the deposition of the middle Eocene Ancón Group, a major uplift of the entire forearc region occurred in the Oligocene. This is supported by the coarse‐grained and immature sediments of the Zapotal Formation that were deposited at 32–30 Ma (Figure , Witt et al, ) along the axis of the restricted Progreso basin. Similar conditions occurred in the Tumbes and Talara depocenters, where coarse‐grained fluvio‐deltaic Oligocene sequences have been identified in the field and in the subsurface (Mancora Formation; Figure , Espurt et al, ; Fernández et al, ).…”

“…Thus, we propose that the deformation of the Azúcar Formation is related to an accretionary event, very likely through reactivation of the ancient Santa Elena accretionary wedge (Figure b), during Late Paleocene‐early Eocene with quiescence during the deposition of the middle Eocene Ancón Group (Figure c), which represents the switch from a trench jump (stage 2) to the reinitiation of a stable subduction system (stage 3) farther to the west. This event is correlated with the end of the cooling and exhumation phase events recorded in the Amotape‐Tahuin massif and is highly coincident with the end of sedimentation of the Azúcar Formation at ~55 Ma (Witt et al, ).…”

“…The stratigraphic sequences deposited above the U2 unconformity are exposed along the coastal cliffs in the Ancón region, north of borehole B2‐ANX1‐1X (Figure ) and have been dated paleontologically and radiometrically as early (?) to middle Eocene (Ordoñez, ; Witt et al, ). We can link the surface exposures of the Ancón Group to the parallel and subparallel seismic facies above unconformity U2 observed on the seismic profile of Figure c.…”

“…The early outer wedge configuration may have developed a west to northwest dipping slope along the seaward margin, where gravity‐driven sediments from the middle Eocene Ancón Group were deposited over unconformity U2, as observed in seismic profiles and the well correlation presented within this study (Figure ). During the Oligocene, a second major uplift of the accretionary prism may have formed the restricted Progreso forearc basin, which is indicated by the deposition of the immature sediments of the Zapotal Formation along the La Cruz fault (Witt et al, ). This continuous uplift of the Santa Elena High, which played a key role in the forearc basin evolution, may have been possible due to the inherited margin configuration, which allowed the accumulation of significant quantities of sediments that prevented an erosional regime of subduction beneath the Santa Elena High and instead created a local outer forearc high since the Oligocene.…”

“…The underlying basement is believed to have been formed of the same complex and deformed sequences of Late Cretaceous to Paleocene age that are observed along the Santa Elena High. After emersion of the Santa Elena High during the late Eocene‐early Oligocene, the Progreso basin began to form as a restricted area with a rapid shift to a continental to shallow marine environment (Witt et al, ). This is supported by the immature sediments of the Oligocene Zapotal Formation (Figure ), which have paleocurrent directions that point toward the center of the basin (Benitez, ).…”