Aeromagnetic Surveys as a Tool for Regional Geological Mapping: A Review of the Case Study of the Southeastern Pampean Ranges, Argentina

Abstract

A geological interpretation of the aeromagnetic survey of the southeastern Pampean Ranges of Argentina –referred to as “Sierras Pampeanas Orientales” in Spanish– , whose basement ages range from late Neoproterozoic to early Paleozoic, was carried out. This interpretation was mostly focused on the identification of the main structural features and the contrast in the magnetic signatures between the different igneous and metamorphic units of the region. The main outcome of the present study is a “solid geology” map that comprises lithomagnetic units or domains constituted by zones with a largely uniform magnetic signature i.e. a locally uniform intensity and gradient of the magnetic field. The lithomagnetic units so defined are bounded by conspicuous changes in the magnetic signatures, often caused by faults and/or fractures. The lithomagnetic units do not fit exactly the exposed lithologic units of the region due to the “transparent” nature of some lithologies to the aeromagnetic method. In addition, the deeper configuration of some units has been investigated by applying an upward continuation filter to the total magnetic field reduced to the pole data; this method was particularly focused on a Lower Paleozoic supracrustal sequence mainly referred to as the San Luis Formation and composed of metasediments. The latter unit, although thin and sub-exposed in some areas, reaches a calculated thickness of ca. 3 km in some sectors of the study area; notably, the latter thickness of metasediments has been interpreted to pertain to a foreland basin developed immediately after the Pampean orogeny, i.e. after ca. 530 Ma. As part of the overall geological interpretation, a deep structural section is also presented, a possible tectonic evolution of the southeastern Pampean Ranges as well being suggested. This evolution would record, after the early Cambrian Pampean orogeny, the inception of a magmatic arc (Famatinian Arc) and an associated back-arc basin during the late Cambrian to early Ordovician, which were later closed and deformed during the collision of the allochthonous Cuyania terrane against the southwestern Gondwana margin in middle Ordovician times.