- MSRB 1302
Zach Eilon will give a talk today entitled Anisotropic seismic tomography within the young Woodlark Rift, Papua New Guinea. The talk is at 2:00 PM at MSRB 1302.
The young Woodlark Rift, Papua New Guinea, is one of the few examples of a rifting continent at the cusp of seafloor spreading. I present tomographic results from the most detailed geophysical study to date of this complex rift. We show that this region contains substantial SKS splitting, a well-established rift at depth (despite limited crustal extension), and unusually large seismic velocity variation due to the juxtaposition of the rift with a relic slab. This rift is well-known for containing the world’s youngest UHP rocks; our tomographic images identify a potential source structure for these rocks, close to the maximum P,T conditions inferred from geothermobarometry. Tradeoffs between velocity and anisotropy heterogeneity complicate the interpretation of differential travel times and have the potential to bias isotropic tomographic models. We formulate a novel method to simultaneously invert differential travel time data and splitting data from teleseismic S-wave arrivals to recover 3-D shear velocity and anisotropy structure. This technique provides a constraint on the depth-extent of shallow anisotropy, otherwise absent from interpretations based on SKS splitting alone. Our work reveals a low velocity rift axis with ≤4% spreading-parallel anisotropy at 50-100 km depth that separates regions of pre-existing lithospheric fabric, indicating the synchronous development of extensional crystallographic preferred orientation and lithospheric thinning. In addition to the insights provided by the anisotropy structure, the improvement in fit to the differential travel time data demonstrates the merit to a joint inversion that accounts for anisotropy.