Does Modern Flat-Subduction Cause Subsidence or Uplift?

Cornell University, Ithaca NY

Investigators

Abstract

Abstract The PI proposes to determine the direction and approximate magnitude of change in long-wavelength elevation that is the consequence of the modern flat subduction system beneath central Chile and western Argentina. The specific study area is the Sierras Pampeanas province, a broad region that is analogous to the Laramide Rocky Mountain province of North America. In both regions basement blocks are uplifted along reverse faults in the foreland of the orogenic belt (Andes Mountains in South America and Cordilleran orogen in North America). But in the North American ancient example the foreland region suffered long-wavelength subsidence due to flat subduction, whereas the Sierras Pampeanas area today stands above sea level and above neighboring foreland regions with more normal subduction angles. The purpose of this study is to test the hypothesis that the Sierras Pampeanas region has risen (at long wavelength) during flat subduction. This project is a study of a continental interior whose objective is to constrain the subduction process. Its utility is tectonic, even if its methods are stratigraphic. Flat subduction merits focused study because it presents an excellent opportunity to view the properties of subducting plates and their interactions with overriding plates from a different perspective, which will shed light on "normal" subduction systems as well as flat-slab systems. The nature of contact between a subducting slab and an overriding plate is of enormous human importance, responsible for the vast majority of the world's seismic budget. But for seismological studies and mechanical models of plate interactions to lead to fruitful insight and accurate predictions, we must have geological constraints on the long-term responses of the overriding plates to the subduction zone processes. The PI proposes to compare modern long-wavelength topography of the Sierras Pampeanas to the topography when flat subduction began using three means of characterizing the middle Miocene paleo-topography. First, a graduate student and the PI will collaborate with Argentine colleagues to determine the extent of middle Miocene marine horizons across the Sierras Pampeanas. Such units are moderately well constrained near the northern and eastern extremes of the region and hypothesized to reach central sectors of the Sierras Pampeanas, but that hypothesis needs critical testing. Second, They will use seismic reflection data (provided by Repsol-YPF) to construct transects of the relief on the basal surface across which the middle Miocene strata onlapped, and then the relief on the surface across which the upper Miocene strata were draped. One transect will span most of the province from north to south, and a second will span the Sierras Pampeanas from west to east. Third, they will take advantage of the fact that a peneplain-like low-relief surface pre-dated the Miocene deposits and use this as a structural marker in both the ranges and basins. They will construct a Digital Elevation Model of current elevations of that pre-Miocene surface and remove from it the short-wavelength deformation across late Miocene to Recent faults. Comparison of the long wavelength elevation variations of this "partially corrected surface" to the modern long-wavelength topography, to the transects of relief underlying the middle and upper Miocene strata, and to the marine shoreline will reveal sectors that uplifted or that subsided during flat subduction.