LATE CENOZOIC TECTONICS AND PALEOMAGNETISM OF NORTH CAUCA BASIN INTRUSIONS, COLOMBIAN ANDES - DUAL ROTATION MODES

Further paleomagnetic results are reported from 18 sites in Late Terti ary hypabyssal igneous rocks adjacent to the Romeral fault system whic h forms the eastern boundary of the North Cauca Basin. The paleomagnet ic directions in these massive igneous intrusions are distributed in v ertical planes parallel to the adjacent Romeral suture zone. Two modes of rotation, involving non-coherent rotations about horizontal rotati on axes and coherent rotations about vertical rotation axes, can expla in the observed remanence patterns. The paleovectors have been rotated variably (non-coherently), both upwards and downwards, through up to nearly 90 degrees, about horizontal axes which are perpendicular to th e prevailing structural trend, These horizontal axis rotations are bel ieved to be associated with shear gradients operating on rough project ions or ''asperities'' on otherwise approximately lenticular bodies. T hese give rise to torques which cause variable horizontal axis rotatio n along splay faults of the Romeral and adjacent systems. In addition, the north segment of the zone of intrusions has been rotated coherent ly, i.e. through uniform angles, of about 30 degrees counterclockwise, about vertical axes relative to the southern zone. The change in decl ination trend corresponds to a major change in trend of the Romeral su ture zone. The vertical-axis rotations are believed to represent regio nal-scale kinking of vertical lenti-laminar fault panels of crust accr eted to the paleocontinental margin along the Romeral suture. It is hy pothesized that oblique transpression has pressed the zone of intrusio ns into the Caldas Re-entrant in the paleocontinental margin.