Paleomagnetic evidence for rapid vertical‐axis rotations during thrusting in an active collision zone, northeastern Papua New Guinea

Abstract

A paleomagnetic study of three thrust sheets of the fold and thrust belt north of the Ramu‐Markham Fault Zone (RMFZ) indicates very rapid vertical‐axis rotations, with differential declination anomalies related to tectonic transport of thrust units. Data from this investigation indicate depositional ages straddling the Brunhes‐Matuyama reversal (780 ka) for the Leron Formation in Erap Valley. Net counterclockwise, vertical‐axis rotations as great as 90° since 1 Ma have occurred locally in the Erap Valley area. These rotations appear to be kinematically related to shear across a tear fault within the foreland fold and thrust belt of the colliding Finisterre Arc, which in turn is aligned with and may be structurally controlled by a major fault in the lower plate. These data indicate that vertical‐axis rotations occurred during thrusting; consequently, the actual rotation rate is likely several times higher than the calculated minimum rate. Such very rapid rotations during thrust sheet emplacement may be more common in fold and thrust belts than is presently recognized. Anisotropy of magnetic susceptibility data yields foliated fabrics with subordinate, well‐grouped lineations that differ markedly in azimuth in the three thrust sheets. The susceptibility lineations are rendered parallel by the same bedding‐perpendicular rotations used to restore the paleomagnetic remanence to N‐S thus independently confirming the rapid rotations. The restored lineations are perpendicular to the direction of tectonic transport, and the minimum susceptibility axes are streaked perpendicular to the lineation. We interpret these anisotropy of magnetic susceptibility data as primary sedimentary fabrics modified by weak strain accompanying foreland thrusting.