EXTENSIONAL BASIN EVOLUTION IN A CONTRACTIONAL OROGEN: AN EXAMPLE FROM THE CORDILLERA BLANCA, PERU

Sedimentary basin analysis, carbonate oxygen isotopic data, and new models of extensional basin development provide insight into the evolution of the Callejon de Huaylas basin adjacent to the Cordillera Blanca, Peru. The Cordillera Blanca is the highest mountain range in the Peruvian Andes, with average elevations > 5000 m. The range is bounded by a large, active, WSW-dipping normal fault striking parallel to active contractional structures located in the eastern Andes. The 170-km-long normal fault is defined by moderate-to-low dips (19°-36° WSW), high footwall topography and moderate extension rates, and has likely accommodated > 10 km of dip slip since the late Miocene. The location of an active normal fault in the thickened hinterland of an active fold-thrust belt leads to questions regarding the evolution of the associated sedimentary basin.

Initial subsidence of the Callejon de Huaylas basin occurred at ~5.4 Ma as a result of normal slip along the Cordillera Blanca detachment fault. The basin is filled by ~1300 m of the upper Miocene-Pliocene Lloclla Formation, which is overlain by Quaternary glacial sediments. The upper 200-400m of the succession record the appearance of granite clasts derived from the Cordillera Blanca batholith in the footwall and a change in sediment dispersal directions from northwest-directed to west-directed. These results suggest that initial unroofing and exposure of the Cordillera Blanca batholith due to fault slip was delayed until later basin development. Upsection dip changes within the basin strata and oxygen isotopic compositions of modern waters and lacustrine carbonates are consistent with syntectonic deposition at high altitude in a basin that experienced intermittent drainage closure. These data also suggest that the Cordillera Blanca normal fault is listric at depth.

Studies of fault propagation suggest that the geometry of sedimentary basin fill is related to fault growth (by fault tip propagation and segment linkage). An extensional basin adjacent to a fault behaving as a single segment will display distinctive sedimentation patterns discernible from those of basins located along segmented faults. Models of basin evolution suggest that the Callejon de Huaylas basin evolved along a large normal fault that behaved as a single segment and propagated in a single direction from north to south.