Geology of the Ayacucho Intermontane Basin, central Peru

The Ayacucho intermontane basin provides the best constraints on the timing and style of Neogene tectonism in Peru. Six deformed unconformity-bound sequences have composite thickness exceeding 8,000 meters. Deposition in an and environment upon gentle topography resulted in the Oligocene or older Ticllas Red Beds. This unit was strongly folded before the deposition of the early Miocene Larampuquio Formation, which marks the onset of Neogene extension and bimodal volcanism. The Quechua I (QI) tectonic event produced erosion and non-deposition, but no discernible folding, in the basin.; The Huayllapata formation, containing conglomerates in a lower member, a series of ∼14.2 Ma bimodal debris flows of the Condoray member, and 12.38 +/- 0.10 Ma conglomerate beds in an upper member, overlies the QI unconformity. Above this unit, the thick Huanta Formation contains alluvial, fluvial, and lacustrine rocks intercalated with intermediate to mafic lava and breccia that together demonstrate extension between about 9.2 and 8.7 Ma. The Huanta Formation and all older units were strongly deformed into an east-vergent thrust duplex during basin closure from the regional QII contraction, which is bracketed between 8.64 +/- 0.05 Ma and 8.76 +/- 0.05 Ma.; The Puchcas volcanics, which overlies the Huanta Formation with strong angular unconformity, was locally deformed between 7.65 +/- 0.10 Ma and 7.77 +/- 0.11 Ma. The Ayacucho Formation, composed volcanic rocks and lesser sedimentary units, generally conformably overlies the Puchcas volcanics. The basin was then deformed again during ca. 5.0 Ma Q III contraction, which preceded eruption of the 3.8 Ma Huari lavas and deposition of the 2.76 +/- 0.03 Ma Cachi Formation. The QIV gently folded the latter unit and was followed by erosion of the Ayacucho basin and incision of the rio Mantaro through the Cordillera Oriental.; The stratigraphy and structure of the Ayacucho intermontane basin demonstrate alternating periods of coupled late Cenozoic extension and contraction. Intense shortening followed extension, volcanism and rapid basin filling. The Quechua tectonic events are discrete, episodic pulses affecting much of the Andes. The pulses are too short to be explained by dip variations in the subducting oceanic plate. Instead, orogen dynamics involving both over thickening and boundary fault coupling may control the regional episodic deformation.