Quechua II contraction in the Ayacucho intermontane basin: Evidence for rapid and episodic Neogene deformation in the Andes of central Perú

In the Ayacucho basin of central Perú the regional Quechua II contractional deformation is bracketed by 40Ar/39Ar isotopic age determinations to a maximum duration of about 300,000 years, and probably less than 150,000 years, centered on 8.7 Ma. The strongly deformed Huanta Formation beneath the Quechua II angular unconformity was deposited during a period of extension that began before 9.05 ± 0.05 Ma. Deposition of a thick succession of alluvial fan deposits interbedded with flows of basaltic andesite in the Tingrayoc Member continued up to about 8.76 ± 0.05 Ma with the later part of the sedimentary record reflected by lacustrine deposits of the Mayocc Member. The upper limit on contractional deformation is constrained by an age of 8.64 ± 0.05 Ma on a unit of tuff near the base of the Puchcas volcanics, which in places was deposited upon near-vertical beds of the Huanta Formation. The Ayacucho Formation was deposited, locally unconformably, upon the Puchcas volcanics beginning slightly before 7.65 ± 0.10 Ma.Extended periods of neutral to tensional stress interrupted by rapid well-developed pulses of contractional deformation demonstrate the episodic behavior of Andean orogeny in Perú. The very short duration for the Quechua II event implies that driving forces for episodic deformation may be related to coupling along the orogen boundaries and strain accumulation and release mechanisms in the continental crust instead of much longer-term variations in the configuration of converging plates.