Vertical axis rotation in the Bolivian orocline, South America: 1. Paleomagnetic analysis of Cretaceous and Cenozoic rocks

Abstract

The results of a paleomagnetic study are reported for 45 new sites or localities in the central Andes of Bolivia and northern Chile (17°S‐23°S), South America, in intrusive, volcanic and sedimentary rocks, ranging in age between ∼7 and ∼80 Ma. These outcrop across the width of the northern and southern limbs of the Bolivian orocline, from the sub‐Andean zone in the east to the volcanic arc in the west. For most localities a mean primary magnetization has been identified which is assumed to reflect a geocentric axial dipole field. A comparison of directions of magnetizations with those predicted by the relevant paleomagnetic reference poles for stable South America reveals declination anomalies which are interpreted in terms of the rotation about a local vertical axis of rigid crustal blocks (hundreds of meters to tens of kilometers across), relative to stable South America. These data, when combined with previous paleomagnetic studies of Upper Cretaceous and Cenozoic rocks, show a pattern of vertical axis rotations which defines three regional domains, extending across the width of the Bolivian Andes. The differences between the various domains are best seen in rocks in the age range ∼20 to 80 Ma; younger rocks acquired a primary magnetization at later stages in the rotation history. For rocks older than 20 Ma, crustal blocks in domain 1, on the northern flank of the Bolivian orocline, have undergone an anticlockwise rotation about a vertical axis in the range ∼8 to ∼33°, with a geometric mean of ∼22; blocks from domain 2 immediately south of the main oroclinal bend have rotated between ∼19° anticlockwise and ∼40° clockwise, with a geometric mean of ∼8° clockwise. Even farther south, in domain 3b, blocks have rotated between ∼3° and ∼57° clockwise, with a geometric mean of 37°. A localised area (domain 3b), in which blocks have undergone ∼32° clockwise rotation in the last ∼65 Myr, lies within the southeastern part of domain 1. The northern boundary of domain 1 and the southern boundary of domain 3a are as yet not well defined. The amount and timing of Cenozoic vertical axis rotation vary across the width of the Andes, and are interpreted to record both regional bending of both limbs of the Bolivian orocline and small block rotation in zones of distributed sinistral or dextral shear during three stages in the Cenozoic: (1) ∼45 to ∼25 Ma, (2) ∼25 to ∼10 Ma, (3) ∼10 Ma to 0. Bending of the Bolivian Andes during the last ∼25 Myr was accommodated, relative to stable South America, by gradients of shortening along the eastern margin of the Andes, particularly in the sub‐Andean zone.