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 A combination of field observations, LA-ICP-MS U-Pb dating on zircon and interpretation of different vintages of unpublished industrial seismic records are used to study the evolution of the forearc system of SW Ecuador including the accretionary prism and the forearc depocenter.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 permit 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 can be dated by U-Pb on zircon provided that the arc is active at the moment of sedimentation. The Azúcar Formation reflects the accretion of SW Ecuador against the South American continental margin from 61 Ma to 55 Ma. This event is post-dated by the formation of an accretionary prism which may have started at circa 55 Ma. Deformation of the accretionary prism may have been less active during sedimentation of the Ancón Group from ~54 Ma (?) to ~40 Ma which shows evidence of normal faulting and other extensional processes. The transition from the Azúcar Formation to the Ancón Group is marked by a significant decrease of the topographic profile, which in turn resulted in sedimentary series that shallow upwards. The forearc basin sensu strictu develops coevally with the uplift of the outer forearc high (Estancia Hills) and the sedimentation of the Zapotal Formation at 32–30 Ma; the latest marking a significant differentiation in the sedimentary style of the area to shallower and even continental environments. From 30 Ma to 10 Ma the forearc basin was defined by shallow water deposits, progressively influenced by tidal dynamics, most likely related to a protected triangular bay symbolizing the shape of the forearc basin. Evidence presented here suggests the existence of a stable outer forearc high limiting the forearc depocenter for at least 20 My and creating, at least in part, the accommodation necessary for an ~3 km thick forearc sedimentary package. The architecture of SW Ecuador is typical of a forearc basin setting including an accretionary prism and an outer forearc high such as those observed in the Kumano (Japan) and the Coast Range and Great Valley basins (USA). The Estancia Hills may correspond to one of the best preserved exposures of an ancient outer forearc high worldwide.