Transient fluvial incision and active surface uplift in the Woodlark Rift of eastern Papua New Guinea

Abstract

The Woodlark Rift of southeastern Papua New Guinea is among the fastest evolving plate boundaries on Earth. Rapid extension led to formation of metamorphic core complexes ahead of the westward-propagating Woodlark basin spreading center, but it is unknown whether all core complexes are still active. We assess the spatial pattern and recent history of rock and surface uplift in the subaerial portion of the Woodlark Rift using stream profile analyses on the D9Entrecasteaux Islands and eastern Papuan Peninsula. Most stream profiles are characterized by prominent convexities, or knickpoints, many of which occur at the heads of inner gorges and likely formed from transient stream erosion in the Quaternary due to an increase in rock uplift rate. The amount of transient incision and rock uplift (∼200–800 m) correlates with channel slopes below knickpoints normalized for drainage area. Recent erosion lags behind rock uplift; hence, the region of study has undergone net surface uplift, elevating a relict landscape upstream of knickpoints by an average of 450 m, and increasing basin relief by an average of 60%. On a local scale, stream slopes, transient incision, and uplifted relict stream channels provide independent quantitative evidence for patterns of active uplift in the D9Entrecasteaux Islands and Papuan Peninsula consistent with available geologic and thermochronologic data. Surface uplift increases from east to west on a regional scale, in a region of active lithospheric extension. Rising topography in thinned crust occurs over low-density asthenosphere, indicating that mantle buoyancy and flow contribute to active surface uplift and landscape evolution in the Woodlark Rift.