PALEOMAGNETIC EVIDENCE FOR RAPID VERTICAL-AXIS ROTATIONS DURING THRUSTING IN AN ACTIVE COLLISION ZONE, NORTHEASTERN PAPUA-NEW-GUINEA

A paleomagnetic study of three thrust sheets of the fold and thrust be lt north of the Ramu-Markham Fault Zone (RMFZ) indicates very rapid ve rtical-axis rotations, with differential declination anomalies related to tectonic transport of thrust units. Data from this investigation i ndicate depositional ages straddling the Brunhes-Matuyama reversal (78 0 ka) for the Leron Formation in frap Valley, Net counterclockwise, ve rtical-axis rotations as great as 90 degrees since 1 Ma have occurred locally in the frap Valley area. These rotations appear to be kinemati cally related to shear across a tear fault within the foreland fold an d thrust belt of the colliding Finisterre Are, which in turn is aligne d with and may be structurally controlled by a major fault in the lowe r plate. These data indicate that vertical-axis rotations occurred dur ing thrusting; consequently the actual rotation rate is likely several times higher than the calculated minimum rate. Such very rapid rotati ons during thrust sheet emplacement may be more common in fold and thr ust belts than is presently recognized. Anisotropy of magnetic suscept ibility data yields foliated fabrics with subordinate, well-grouped li neations that differ markedly in azimuth in the three thrust sheets. T he susceptibility lineations are rendered parallel by the same bedding -perpendicular rotations used to restore the paleomagnetic remanence t o N-S thus independently confirming the rapid rotations. The restored lineations are perpendicular to the direction of tectonic transport, a nd the minimum susceptibility axes are streaked perpendicular to the l ineation. We interpret these anisotropy of magnetic susceptibility dat a as primary sedimentary fabrics modified by weak strain accompanying foreland thrusting.