90-6 “Top down” insight into hypothesized flat-slab subduction and underthrusting processes

Abstract:

The Late Cretaceous–Paleogene North American Laramide is a classic zone of intra-plate deformation, with localized deformation occurring >1000 km from the paleo-plate boundary. This orogen has traditionally been interpreted to result from flat east-dipping subduction of the Farallon slab that exerts basal tractions on the upper plate, but alternative models argue for plate-margin end-loading or continental indentation to drive intra-plate deformation. Similar arguments have been made for the Mesozoic SE China (Yanshanian) orogen associated with the west-dipping Paleo-Pacific (Izanagi) plate. Numerous bottom-up geophysical surveys and numerical simulations have been conducted to probe these geodynamic scenarios, but in the theme of this session, here we take the top-down approach of examining upper-plate geologic records in the hinterland regions to investigate these processes. Our goals are not to definitively arrive at a particular solution but instead discuss important observations that any viable model should explain. Both orogens involved inboard and outboard sweeps of magmatism that are interpreted to track shallowing and steepening of the subducting slab, although alternative explanations invoke slab foundering or breakoff. Notably, both orogens involved wide expanses of crust between the plate boundary and intraplate orogen that did not experience strong contractional deformation during the phase of postulated flat-slab subduction or indentation. Nevada was a quiescent highland with limited Cretaceous shortening and local extension, that became dominated by Eocene lake systems. The SE China hinterland experienced shallow marine sedimentation that has been attributed to downward flat-slab pull. These hinterland regions both experienced peak temperature conditions and widespread porphyry copper mineralization during this time, which reflects the heat and fluids transported into the upper plate. In particular, the hot, weak, and negligibly deforming hinterland regions between the plate boundary and intra-plate orogens appear inconsistent with simple plate-boundary-driven models (end loading or indentation), which would be expected to also deform this intervening weak crust. A strong lithospheric mantle or elevated gravitational potential energy could explain the hinterland quiescence for such plate-boundary-type models. Consideration of these geological observables and comparisons to analogous orogens can help resolve some of these open questions. In summary, the geologic record is inherently noisy and incomplete, which can lead to real preservation and confirmation biases, but bottom-up approaches that do not fully engage with geologic observations are no more than wishful thinking.

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