FOLD ANALYSIS OF THE CENTRAL PERUVIAN SEGMENT OF THE ANDEAN OROGENY REVEALS STRAIN ACCUMULATION PATTERNS DURING REPEATED PARALLEL CONTRACTIONAL EVENTS

Regional mapping of folds in the Peruvian Andes, assembled from field observations and interpretation of INGEMMET 100k-scale geologic maps, aerial photography and satellite imagery, document 3,358 fold hinge lines and 1,753 faults. Systematic measurement of folds and faults between 8 and 14 degrees south latitude are used to derive the areal density of these structures. The contoured data define four structural domains of more tightly spaced structures: from west to east, the Ticlio, Huanca, Satipo, and Frontal domains. The folds and faults are the result of repeated contractional events in the Andean orogeny, termed phases by Steinmann (1929). These shortening phases are the Incaic I and II between 65 and 45 Ma (Noble and Wise, 2016), the Aymara event at about 25 Ma, and the three Quechua phases at 17.5 Ma, 8.7 Ma (Wise et al., 2008), and 5.4 Ma. Isotopic age and geologic field controls are insufficient at a regional scale to separate these events using cumulative map patterns. Mégard (1978) suggested that deformation, beginning with the Incaic phases, started in the west and migrated progressively towards the east. Despite being often cited, this concept is largely unsupported; Incaic deformation could have taken place concurrently over the entire orogen and Quechua deformation is found at various localities from east to west. The spacing of folds indicates that wavelengths are shorter in Mesozoic and Paleozoic sedimentary rocks than in younger Cenozoic volcanic rocks. This contrast in fold wavelengths represents a combination of the older rocks having a greater amount and number shortening events and also the nature of rock competency. Most of the Incaic deformation predates the Cenozoic volcanic rocks, and deformation by the Aymara and Quechua phases are increasingly more spatially restricted. Deep-seated features within the Ticlio and Huanca structural domains – perhaps coinciding with a zone or zones of asthenosphere upwelling - controls the location of Miocene intrusion-related mineral districts (Antamina, Morococha, Yauricocha, Cerro de Pasco, and Huancavelica-Santa Bárbara) and the position of modern hot springs.