Magnetostratigraphy of the Miocene Corque Basin, Bolivia: Implications for the geodynamic evolution of the Altiplano during the Late Tertiary

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Abstract
A magnetostratigraphic study of a thick Miocene continental red bed sequence (Totora Formation) located in the north central Altiplano (the Corque syncline) yielded quantitative determinations of the sedimentation rate and better constraints of the timing of the deformation within the Altiplano. Paleomagnetic results obtained in 653 samples from a composite 4.5 km thick red bed sequence across the Corque syncline indicate that most of the Totora sequence was deposited in the middle Miocene from 14 to 9 Ma with high sedimentation rates up to 970 m/Myr from 12 to 9 Ma. The high rate of infill of the Corque basin demonstrates active subsidence of the northern Altiplano during the middle Miocene. Deformation ceased in the northern Altiplano domain only at the end of the Miocene. Two major tuff beds within the sequence have been previously dated by 39Ar/40Ar of sanidine minerals [Marshall et al, 1992]. The excellent agreement between the magnetostratigraphic dating of the tuffs and the radiometric dating independently supports the age calibration of the geomagnetic reference timescale proposed by Cande and Kent [1995]. The numerous paleomagnetic results obtained in middle Miocene sediments within the Corque syncline, a structure which can be traced for more than 100 km along its axis, demonstrate that the Corque syncline rotated counterclockwise by 10.8°±2.9° since 9 Ma as a consequence of the internal deformation of the Altiplano. Inclination flattening of 17° is recorded in these red bed sediments. This result provides additional evidence that detrital magnetizations, especially in red beds, should be taken with caution when estimating paleolatitudes or long‐term variations of the geomagnetic field.