Abstract
The Bolivian Andes are an archetypal convergent margin orogen with a paired fold-thrust belt and foreland basin. Existing chronostratigraphic constraints highlight a discrepancy between unroofing of the Eastern Cordillera and Interandean Zone fold-thrust systems since 40 Ma and the onset of rapid sediment accumulation in the Subandean Chaco foreland after 11 Ma, previously attributed to Miocene climate shifts. New results from magnetostratigraphic, backstripping, erosional volumetric calculations, and flexural modeling efforts are integrated with existing structural and thermochronologic datasets to investigate the linkages between shortening, exhumation, and subsidence. Magnetostratigraphic and backstripping results determine tectonic subsidence in the Chaco foreland basin, which informs flexural models that evaluate topographic load and lithospheric parameters. These models show that Chaco foreland subsidence is consistent with a range of loading scenarios. Eroded volumes from the fold-thrust belt were sufficient to fill the Chaco foreland basin, further supporting the linkage between sediment source and sink. Erosional beveling of the Eastern Cordillera, local intermontane sediment accumulation after 30–25 Ma, and regional development of the high-elevation San Juan del Oro geomorphic surface from 25 to 10 Ma suggest that the western Eastern Cordillera did not store the large sediment volume expected from erosion of the fold-thrust belt, which arrived in the Subandean Zone after 11 Ma. Eocene to middle Miocene foreland basin accumulation was likely focused between the Eastern Cordillera and Interandean Zone, and has been almost completely recycled into the modern Subandean foreland basin. The delay between initial fold-thrust belt exhumation (early Cenozoic) and rapid Subandean subsidence (late Cenozoic) highlights the interplay between protracted shortening, underthrusting, and foreland basin recycling. Only with sufficient crustal shortening, accommodated by eastward advance of the fold-thrust belt and attendant underthrusting of Brazilian Shield lithosphere beneath the Subandes, did the Subandean zone enter proximal foreland basin deposystems after ca. 11 Ma. Prior to the late Miocene, the precursor flexural basin was situated westward and not wide enough to incorporate the distal Subandean Zone. These results highlight the interplay between a range of crustal and surface processes linked to tectonics and Miocene climate shifts on the evolution of the southern Bolivian Andes.
Highlights
Paired fold-thrust belts and foreland basins in retroarc contractional systems archive the interactions between convergent plate boundary dynamics and surface processes spanning deep time to the present
Regardless of remaining challenges with modeling and reconstructions, these results demonstrate that the tectonic subsidence observed in the southern Bolivian Chaco foreland are consistent with flexural subsidence predicted from a range of plausible Eastern Cordillera and Interandean Zone topographic loads
We propose that the large magnitude of shortening and attendant lateral translation of the fold-thrust belt, coupled with flexural subsidence over a weak lithosphere, created an initial foreland basin that was too narrow to induce sediment accumulation in the distal Subandean Zone
Summary
Paired fold-thrust belts and foreland basins in retroarc contractional systems archive the interactions between convergent plate boundary dynamics and surface processes spanning deep time to the present. The Eastern Cordillera recorded protracted shortening spanning the Eocene to Miocene (McQuarrie, 2002; Barnes et al, 2006, 2008; Gillis et al, 2006; Ege et al, 2007; Anderson et al, 2018), Subandean basin fill recorded punctuated sediment accumulation since the middle to late Miocene, possibly attributable to climate shifts (Strecker et al, 2007) This contrast highlights a mismatch between active deformational phases in the Eastern Cordillera fold-thrust belt and the main phase of subsidence in the accompanying foreland basin (Echavarria et al, 2003; Uba et al, 2006, 2009; Calle et al, 2018)
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