Reconciling Spatial and Temporal Patterns of Cenozoic Shortening, Exhumation, and Subsidence in the Southern Bolivian Andes

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.

REGIONAL SUBSURFACE MAPPING AND 3D FLEXURAL MODELING OF THE OBLIQUELY-CONVERGING PUTUMAYO FORELAND BASIN, SOUTHERN COLOMBIA

The Putumayo foreland basin (PFB) is an underexplored, hydrocarbon-bearing basin located in southernmost Colombia. The PFB forms a 250-km long segment of the 7000-km-long corridor of Late Cretaceous-Cenozoic foreland basins produced by eastward thrusting of the Andean mountain chain over Precambrian rocks of the South American craton. We use ∼4000 km of 2D seismic data tied to 28 exploratory wells to describe the basin-wide structure and stratigraphy of an underexplored hydrocarbon basin. Based on seismic interpretation and comparison with published works from the southward continuation of the PFB into Peru and Ecuador, three main across-strike, structural zones include: 1) the 20-km-wide, Western structural zone closest to the Andean mountain front characterized by inversion of older, Jurassic half-grabens during the late Miocene; 2) the 45-km-wide, Central structural zone characterized by moderately-inverted Jurassic half-grabens; and 3) the 120-km-wide, Eastern structural zone characterized by the 40-km-wide, N-S trending Caquetá arch. The five mainly clastic tectonosequences of the PFB include: 1) Lower Cretaceous pre-foreland basin deposits; 2) Upper Cretaceous-Paleocene foreland basin deposits; 3) Eocene foreland basin deposits related to the early uplift of the Eastern Cordillera; 4) Oligocene-Miocene underfilled, foreland basin deposits; and 5) Plio-Pleistocene overfilled, foreland basin deposits. We used 3D flexural modeling to identify the elastic thickness (Te) of the lithosphere below the PFB, in order to model the location of the sedimentary-related and tectonically-related forebulges of Cretaceous to Oligocene age. Flexural analysis shows two pulses of rapid, foreland-related subsidence first during the Late Cretaceous-early Paleocene and later during the Oligocene-Miocene. Despite the present-day oblique thrusting of the mountain front, flexure of the PFB basement has produced a tectonic forebulge now located in the Eastern structural zone and controls a basement high that forms the eastern, updip limit for most hydrocarbons found in the PFB.

Linking deep-Earth processes, basin stratigraphy, and topographic build-up during the Neogene Zagors orogeny in the Kurdistan region of Iraq

<p>Plate tectonics can lead to construction of mountain belts, whereas surface processes destruct the orogenic masses and redistribute the surface load. These processes can be modulated by climate through variation in air temperature and the magnitude-frequency distribution of precipitation. In the northwestern Zagros orogenic belt the driving force for hinterland uplift has been baffling. The key concern is whether uplift is due to upper crustal shortening and related crustal thickening (local uplift) or to deep lithospheric processes (regional dynamic uplift) such as slab breakoff and/or to lithospheric mantle delamination. The stratigraphic record is sensitive to geodynamic processes, yet distinguishing the tectonic signatures from the climate-induced signatures is necessary. The goal of this research is to test these competing mechanisms of orogenesis through field-based evaluations of shifts in foreland basin stratigraphy, provenance, detrital geochemistry, and climate change through time as well as flexural modeling for the northwestern Zagros orogenic belt. In the Kurdistan region of Iraq, the northwestern Zagros orogenic belt is characterized by a well preserved ~4 km thick stratigraphic column of the Neogene synorogenic predominantly clastic continental deposits that coarsen and thicken upwards: The Fatha (middle Miocene), Injana (late Miocene), Mukdadiya (latest Miocene), and the Bai-Hasan (Pleistocene) Formations. These units, in addition to sandstone beds, include thick poorly consolidated mudstone packages that in some places reach ~100 m. Preliminary results show that the frequency and thickness of sandstone-filled channels increases upsection, leading to an amalgamation of sandstone packages towards the top. This thickening-upward trend was additionally associated with an increase in the grain size. These patterns of stratigraphy dynamics hint to a progradation of the depositional systems, driven either by an increase in the sediment flux relative to the subsidence rate, or by a propagation of the orogen front towards the foreland basin. Sm-Nd analysis on the fine material packages revealed a crustal origin (εNd-) comparable to the Arabian shield, with an older crustal age upsection. Weathering proxy data such as chemical index alteration (CIA) and K<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub> ratio yield evidence for a weathering intensity that increases upsection. X-Ray diffraction data from the clay-size materials (<2-μm) show contents of smectite, illite, kaolinite and Fe-rich chlorite, with an increasing abundance of smectite minerals upsection. These mineral assemblages demonstrate a semi- arid/humid climate likely with an increasing seasonality through time, which could possibly have resulted in an increasing sediment flux. Furthermore, basic flexural modeling for the northwestern Zagors orogenic belt indicates that the present-day Zagros topography, and thus topographic load alone, cannot explain the observed basin depth. Overall, these evidences suggest that exhumation of the source terranes was enhanced by increased weathering, yet a geodynamic process could have been the main driver for controlling the formation of accommodation space and uplift of the mountain belt.</p>