Late cenozoic tectonics and paleomagnetism of North Cauca Basin intrusions, Colombian Andes: Dual rotation modes
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1996-09-15
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ELSEVIER SCIENCE BV
Resumen
Further paleomagnetic results are reported from 18 sites in Late Tertiary hypabyssal igneous rocks adjacent to the Romeral fault system which forms the eastern boundary of the North Cauca Basin. The paleomagnetic directions in these massive igneous intrusions are distributed in vertical planes parallel to the adjacent Romeral suture zone. Two modes of rotation, involving non-coherent rotations about horizontal rotation axes and coherent rotations about vertical rotation axes, can explain the observed remanence patterns. The paleovectors have been rotated variably (non-coherently), both upwards and downwards, through up to nearly 90°, about horizontal axes which are perpendicular to the prevailing structural trend. These horizontal axis rotations are believed to be associated with shear gradients operating on rough projections or "asperities" on otherwise approximately lenticular bodies. These give rise to torques which cause variable horizontal axis rotation along splay faults of the Romeral and adjacent systems. In addition, the north segment of the zone of intrusions has been rotated coherently, i.e. through uniform angles, of about 30° counterclockwise, about vertical axes relative to the southern zone. The change in declination trend corresponds to a major change in trend of the Romeral suture zone. The vertical-axis rotations are believed to represent regional-scale kinking of vertical lenti-laminar fault panels of crust accreted to the paleocontinental margin along the Romeral suture. It is hypothesized that oblique transpression has pressed the zone of intrusions into the Caldas Re-entrant in the paleocontinental margin.