Rift Structure in Eastern Papua New Guinea From the Joint Inversion of Receiver Functions and Seismic Noise

Abstract:

The recent CDPapua seismic array in the active D’Entrecasteaux-Woodlark Rift provides insights into how continental crust accommodates large extension. Here, >100 km of extension has occurred in the last 4-6 Ma, exhuming rocks from 100 km depth. To better understand the modes of deformation of the crust, we analyze shear wave velocity (Vs) distribution for a set of temporary land and ocean bottom broadband stations. We resolve the depth of the main velocity contrasts using receiver function (RF) analysis, alleviating the intrinsic trade-off between depth and velocity intrinsic by joint inversion with dispersion constraints (10 – 100 s) from earthquake surface waves and ambient noise. A transdimensional Bayesian scheme explores the model space (Vs in each layer, number of interfaces and their respective depths), minimizing the number of layers required to fit the observations given their noise level. Preliminary results suggest that the Moho is sharp in most places, with a depth of 28-38 km and 20-27 km below the Papuan Peninsula and the highly-extended D’Entracasteaux Islands, respectively. The mid-lower crust of these regions appears to be similar and consistent with felsic compositions, 3.25≤Vs≤3.5 km/s, and may represent the Owen-Stanley Metamorphic Belt or underlying continental rocks. A fast layer (3.75≤Vs≤4 km/s) is observed below the Papuan Peninsula in the 20-30 km depth range and may indicate more mafic lower crust. In contrast, faster velocities between 10 and 20km depth are modeled below the Goodenough Basin (3.75≤Vs≤4 km/s) and the Trobriand Basin (3.5≤Vs≤3.75 km/s) where rocks of the Papuan Ultramafic Belt have been suggested, although these results partly depend upon complicated signals from ocean-bottom seismometers. Well-located seismicity shows that active fault systems generally follow the boundaries between regions of different crustal velocity structure. Overall these results confirm a continental velocity structure for the onshore parts of the rift, but allow for much more mafic crust beneath intervening basins. Much of the rifting at crustal depths could have been accommodated by opening these basins.