Lower Paleozoic relative motion of the Arequipa Block and Gondwana; Paleomagnetic evidence from Sierra de Almeida of northern Chile

Abstract

Paleomagnetic results have been obtained from a suite of Paleozoic samples from Sierra de Almeida within the high eastern portions of the Atacama desert. Characteristic directions are discussed for two sequences of pre‐Silurian lavas of probable Cambro‐Ordovician age, the Late Cambrian Choscas pluton, three Late Ordovician‐Early Silurian plutons, the Devonian‐Carboniferous Lila Formation and late Paleozoic volcanic units of the Pular and Cas Formations. The Choschas pluton and one lava series yield similar northerly and shallow directions which for the presence of reversals and their concordance are suggested to represent early Paleozoic Arequipa plate directions. Directions in the three Late Ordovician‐Early Silurian plutons pass a tilt test using an overlying erosional unconformity, and these also include reversals. Together with directions from a second lava series (a roof pendant in an Early Silurian pluton) these define poles compatible with Silurian Gondwana results from Africa and Australia. In situ directions from the basal red beds of the Devonian Lila Formation are inconsistent with Devonian Gondwana or stable South American Poles, but (like the Devonian strata of the Appalachians) they are consistent with a tilt‐corrected overprint of Kiaman Superchron age. These results, together with previous results from the late Paleozoic Cas and Pular Formations are discordant from the Gondwana path only for the latest Cambrian‐earliest Ordovician. The discordance in paleomagnetic data, together with regional geologic constraints, can be explained by a model in which the Arequipa block, representing a paraautothonous finger of Gondwana (like Japan or the Iberian Peninsula) rotated about a nearby pole but was then resutured during the Silurian. Such a scenario resolves much of the discrepancies in the models which have emerged from Peru, Chile, and Argentina.

Reinterpretation of the Ordovician rotations in NW Argentina and Northern Chile: a consequence of the Precordillera collision?

“…Paleomagnetism is an excellent method to establish the paleolatitudinal position and former orientation of crustal blocks. Early Paleozoic paleomagnetic data from NW Argentina and N Chile have shown large systematic rotations with limited to no paleolatitude anomaly (Valencio et al 1980;Forsythe et al 1993;Conti et al 1996;Rapalini et al 1999Rapalini et al , 2002Rakotolofo 2004;Spagnuolo et al 2008;Spagnuolo 2009). These paleomagnetic poles were obtained in the Western Puna of Chile and Argentina, the Eastern Puna, the NW Pampean Ranges and the Famatina Ranges.…”

“…Counterclockwise rotations (Western Puna) Forsythe et al (1993) carried out a paleomagnetic study in the Western Puna of Chile. Paleomagnetic poles were calculated from the Late Cambrian (ca.…”

“…1 Previous models to explain the Ordovician rotations. a Reconstruction of the Arequipa rotation, opening a basin during the Precambrian-Cambrian and closing it during the Ordovician-Silurian (Forsythe et al 1993). b Paleogeographic reconstruction of Gondwana and Laurentia showing the position of the composed terrane of Famatina-Eastern Puna volcanic arc (Conti et al 1996) …”

“…Published tectonic interpretations of these paleomagnetic results (Forsythe et al 1993;Conti et al 1996) proposed the opposite rotations of large para-authocthonous terranes (i.e. ccw for the Arequipa-Antofalla and cw for the Eastern Puna-Famatina) producing the closure of ocean floored backarc basins to the east and their subsequent accretion to the Gondwana margin ( Fig.…”

La Formacion QuebradaGrande, del Ordovicico, Cordon de Lila (Region de Antofagasta, norte de Chile): significado estratigrafico y paleogeografico.

“…Since the first paleomagnetic data presented by Forsythe et al (1993), the Proterozoic rocks of northern Chile-southern Peru (the 'Arequipa-Antofalla Massif'; Ramos, 1988) have been considered as parautochthonous with respect to Gondwana. The clockwise and counterclockwise rotation movements of this block opened and closed a small oceanic basin between this terrane and the Gondwana margin.…”