Abstract: Measurement of Surface Uplift Rate During Collision-Induced Basin Inversion in Northern Papua New Guinea

L. D. Abbott, E. A. Silver, R. S. Anderson, J. Galewsky, R. Smith, J. C. Ingle, S. Kling, D. W. Haig, W. Sliter

The Finisterre Mountains of Papua New Guinea, which have been uplifted in the last 3 Ma by an arc-continent collision, offer a unique opportunity to measure the "surface uplift rate" in a collisional mountain belt. "Surface uplift rate" is a measure of the uplift rate of the average topographic surface. It is the only true measure of purely tectonically-driven uplift. It is a difficult quantity to determine because it requires preservation of the youngest rocks in the depositional sequence and knowledge of their age and original depositional elevation. Typically, such rocks are eroded soon after uplift begins. In the Finisterre Range, the Late Pliocene to Early Pleistocene marls that form the top of the depositional sequence are preserved as the caprock of an extensive pl teau, allowing calculation of the surface uplift rate. These rocks were deposited at greater than 2000 m water depth and now crop out at elevations over 2000 m above sea level, attesting to their rapid uplift.

We have collected samples of this marl and the underlying limestone from 15 locations spanning the 115 km length and 30 km width of the plateau. Biostratigraphic and strontium isotope analyses have allowed us to date the rocks and determine their depositional water depth. From these data we have constructed geohistory diagrams that quantify basin subsidence and subsequent uplift during collision-induced basin inversion. By averaging the uplift rates calculated at each sample location we have calculated the regional uplift rate of the entire plateau.

Several rivers dissect the plateau. Removal of material from these river gorges has led to erosionally-driven isostatic uplift, which must be accounted for in order to calculate the tectonically-driven surface uplift rate. Interpolation of the original depositional surface between the large plateau remnants on either side of the gorges allows us to calculate the amount of erosion, and from this the magnitude of erosionally-driven uplift. Subtraction of this value from the regional uplift results in a value for the purely tectonic surface uplift rate.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California