CRUSTAL THINNING BENEATH ACTIVE METAMORPHIC CORE COMPLEXES, WOODLARK RIFT, PAPUA NEW GUINEA

Metamorphic core complexes expose mid-crustal rocks in their footwalls implying removal of upper crust; yet lie at regionally high elevation. Seismic refraction studies in the Basin-and-Range show that the Moho there beneath core complexes is flat, requiring either extensive underplating or lower-crustal flow, properties often inferred as necessary features of core complexes. However, results from a new seismic experiment in the Woodlark Rift show that a flat Moho is not universal, and that in some cases their unroofing can be accompanied by comparable elevation of the Moho. In 1999-2000, we deployed 19 broadband seismographs across the Woodlark Rift of Papua New Guinea, one of the world's most rapidly extending rifts (20-30 mm/yr locally). Using receiver function techniques, we estimate crustal thickness and depths at several different azimuths from each station. These measurements reveal remarkable variations in crustal thickness - crust beneath the core complexes is 10-15 km thinner than beneath adjacent less-extended areas. This result shows that lower-crustal flow (or extensive underplating) need not compensate metamorphic core complexes. Perhaps, the short time since onset of exhumation here is insufficient for such flow to occur: some of the exposed mid-crustal rocks were at 4-5 kbar (~15 km depth) as recently as 2-2.5 Ma, and extension commenced only 5-6 Ma ago. If so, these observations place a lower bounds on lower-crustal viscosity. In any case such rapid thinning is likely a prerequisite for formation of new ocean basins, as appears to be happening here. The result does present a problem, however, as regional topography lies near sea level in both extended and unextended regions despite Moho relief. Preliminary inversion for upper-mantle P-wave velocity shows large variations across strike (0.4 km/s changes), with the lowest Pn velocities appear offset north (downdip) from the core complexes. The low velocities may correspond to the low densities necessary to elevate the core complexes, although they are offset. The offset reveals a fundamental asymmetry of extension, consistent with throughgoing detachment systems.