90-5 Dynamics of Flat-Slab Subduction and the Effects on the Overlying Continent

Abstract:

Transient episodes of flat-slab subduction have profoundly shaped the geological record of western North and South America. In the western United States, the development of Farallon flat-slab subduction during the Late Cretaceous drove an inboard migration of magmatism and basement-involved deformation to more than 1500 km from the plate margin. Subsequent foundering of the flat slab during the Late Oligocene was accompanied by widespread magmatism and rapid changes in surface topography. Although the first-order surface expressions of flat-slab subduction are well documented, its effects on the deep continental lithosphere remain poorly understood.

We use two-dimensional numerical models based on the tectonic setting of the western United States to investigate how flat-slab subduction modifies the thermal, mechanical, and compositional structure of the overlying continent. Models first examine flat-slab development, in which slab shallowing is driven by continental advance and subduction of anomalously buoyant oceanic crust (the Conjugate Shatsky Rise). As the slab flattens, it mechanically entrains the lowermost 20-50 km of the continental lithosphere, forming a keel that thickens as it is pushed inland. Flat-slab emplacement progressively cools the overlying lithosphere, suppressing magmatism, while continued slab dehydration metasomatizes the deep continent.

Slab removal is driven by an increase in oceanic plate density and/or a slowdown in continental advance. Models explore two modes of removal: (1) rollback as the slab progressively steepens and (2) vertical foundering following slab breakoff at the seaward edge of the flat segment. Both modes produce changes in surface topography, continental shortening, and magmatism, although the spatiotemporal patterns and magnitudes differ. The models predict that hydration of the continental mantle lithosphere during the flat-slab episode primes it for extensive melting following slab removal. In addition, the former flat-slab region is characterized by thinned continental lithosphere resulting from both earlier mechanical displacement and subsequent gravitational thinning. Our results demonstrate that flat-slab subduction can significantly modify continental lithosphere structure and that these modifications are consistent with present-day geophysical observations of the western United States lithosphere.

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