Abstract

Strata of the Neogene Ayacucho intermontane basin, central Peru, can be subdivided into three tectonostratigraphic units: the Larampuquio volcanics, consisting largely of intermediate lavas and coarse conglomerates of similar lithology; the unconformably overlying Huanta Formation, consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas, tuffs, and conglomerates; and the unconformably overlying Ayacucho Formation, consisting of complexly interfingering volcaniclastic sedimentary rocks, silicic tuffs, and lavas of intermediate to silicic composition.

Radiometric ages of 17.3 ± 0.2 m.y. and 18.3 ± 0.6 m.y. have been obtained on tuffs from the upper and lower parts of the Larampuquio volcanics. Determinations on 2 rocks from the lower member of the Huanta Formation indicate an age of ∼11.4 ± 0.5 m.y. and a rock from the upper member is dated at 9.3 ± 0.3 m.y. B.P. Units of the Ayacucho Formation yield ages showing that it was deposited between ∼7.7 and 6 m.y. ago. The intervening Molinoyocc and Puchcas volcanics were erupted between ∼9.9 and 7.4 m.y. ago; these units may postdate the Huanta Formation or may be associated in part with the Huanta and in part with the Ayacucho Formation.

Coarse conglomerates of the Larampuquio volcanics represent debris eroded from highlands uplifted during the first, and most intense, pulse of the late Cenozoic “Quechuan” phase of Andean compressive tectonism that began in the early Miocene. Folding of the Larampuquio volcanics may have taken place during this, or a subsequent, pulse of compression. A marked unconformity between the Huanta and Ayacucho Formations demonstrates a period of deformation between ∼8.5 and 9.5 m.y. ago. Compression also was active during at least part of the deposition of the Ayacucho Formation, as shown by inter-fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation. Folded rocks of the Ayacucho Formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3.8 ± 0.4 m.y. B.P. and ∼5.5 m.y. B.P., bracketing a well-defined pulse of deformation slightly younger than 6 m.y.

These subsequent episodes of compressive deformation represent recurrent pulses of the “Quechuan” phase. Folding related to these late Quechuan pulses is restricted in the Andes to northwest-trending belts corresponding to reactivated fault zones in the pre-Neogene substratum. A system of north-trending dikes in the Huachocolpa district 70 km west of Ayacucho suggests relaxation of compression in the Pliocene, and compression of Quaternary age has been recognized in the Huancayo area 170 km northwest of Ayacucho. Taken together with the first pulse of Quechuan deformation in the early and middle Miocene, these data indicate at least four pulses of crustal compression during the late Cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension.

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