Dynamic subsidence in the Colorado basin, offshore Argentina, South Atlantic

Abstract

The Colorado basin in the SW Atlantic is one of the key exploratory frontier basins along the Argentine shelf. Thestratigraphy shows a typical rift basin, divided into two major seismic successions, a Jurassic-Lower Cretaceoussynrift followed by an Upper Cretaceous to Cenozoic postrift. They represent, respectively, the breakup of Gondwanaand passive margin evolution. While the synrift basin formation was associated with the development ofstructural grabens, the preservation of thick (thousands of meters) postrift sequences are still debated becausethey cannot be accounted for only by thermal subsidence. In this work, we conducted a multimethodological subsidenceanalysis across the Colorado basin, including part of the continental slope. We performed a 1D backstrippinganalysis on 10 hydrocarbon exploration wells and 32 pseudo-wells, covering the whole basin area and a uniformstretching model, which were supported using dynamic topography data from the study region. From thebackstripping analyses, considering the six major postrift sedimentary successions described in the literature, wecould recognize three subsidence stages. An initial rapid subsidence stage during the post-rift (125–83 Ma),which is restricted to the main depocenters, followed by a stage of generalized and low subsidence focused on themain axis of the basin (83–33 Ma), with an increase in the last 33 my (33–0 Ma). The stretching models exhibit atypical asymptotic curve, depicting significant subsidence at the onset of the postrift stage, gradually diminishingto minimal values within a few tens of millions of years. Upon comparing the backstripping curves with thestretching subsidence models, particularly during the final flat segments, a residual negative value emerges overtime (locally exceeding −500 m), indicating considerable residual subsidence during the Cenozoic postrift evolutionalong the Colorado Basin. This could be interpreted as dynamic subsidence. This residual sinking in the Argentinashelf matches with dynamic topography numerical models in the study region as well as the occurrenceof accumulated slabs in the mantle detected by seismic tomography studies. We propose that the accumulation ofslabs along this segment of the South American subduction zone might be the drivers of dynamic subsidence,which assisted to the preservation of a large thickness of post-rift strata along the SW Atlantic margin.