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Phanerozoic evolution of subduction below the Americas:
A combined analysis of plate reconstructions and tomography

Abstract

Due to the lack of constraining data, plate tectonic reconstructions become more and more uncertain when going far back in geologic time. Constraints from seafloor spreading become scarce or are absent, geologic data has been overprinted and paleomagnetism, although the most accurate technique to constrain the paleoposition of tectonic blocks, is not able to resolve longitude. Therefore, and in particular for the period before 130 Ma, large uncertainties exist in the location of tectonic blocks and plates through geological history.
Recent advances in tomography have made it possible to visualize the seismic velocity structure of the upper and lower mantle with increasing detail. Subducted lithosphere is now seismologically detectable as regions with relatively fast seismic wave speeds, primarily because of its lower temperature in respect to the ambient mantle. The subducted slabs, made visible by tomography, preserve a record of former plate convergence.
In this way an additional and independent source of information is provided to test the tectonic reconstructions. The other way round, plate tectonic models can help to develop new interpretations for imaged mantle structure.
In this thesis, the tomographic model of Bijwaard et al. %5B1998%5D is used with reconstructions from ExxonMobil for a combined analysis of both mantle structure and plate motions associated with the plate-tectonic evolution of North and South America.
Fourteen wave speed anomalies have been identified within the lower mantle and associated with past subduction.
The oldest anomalies lie at the base of the mantle and are Ordovician-Permian in age and are associated with the closure of the Iapetus Ocean and subduction of paleo-Pacific, or Panthalassic oceanic lithosphere.
It follows from the interpretation that he average sinking rate in the lower mantle is about 1 cm/yr.
Further, new inferences are derived that may help resolve several geological problems. These problems include the long-debated Origin of the Caribbean, the Baja British Columbia controversy, the Laramide Orogeny and their relation with the evolution of the oceanic Farallon and Kula plates. Furthermore, it is proposed that during the Early-Mid Cretaceous most consumption of the Farallon plate took place at intra-oceanic subduction zones, 1000-2000 kilometres west of the North American margin. An Early Cretaceous break-up of the Farallon plate is therefore necessary.