HR: 08:15h
AN: T51E-02    [Abstracts]
TI: Seismic Imaging of the Transition from Continental Rifting to Seafloor Spreading, Woodlark Rift System, Papua New Guinea
AU: * Ferris, A
EM: aferris@bu.edu
AF: Boston University, 685 Commonwealth Ave, Boston, MA 02215 United States
AU: Abers, G A
EM: abers@bu.edu
AF: Boston University, 685 Commonwealth Ave, Boston, MA 02215 United States
AU: Taylor, B
EM: taylorb@hawaii.edu
AF: University of Hawaii, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Zelt, B
EM: bzelt@iodp-mi-sapporo.org
AF: Hokkaido University, N21, W10, Sapporo, 001-0021 Japan
AB: The Woodlark rift system of Papua New Guinea is one of the few active examples of complete continental rifting. In continental crust north-south extension accommodates 25-40 mm/yr of extension, and extension rates increase eastward so that full seafloor spreading occurs. We report on the results from the first passive seismic survey in the region, an onshore/offshore deployment covering a 360x280 km region surrounding the active spreading tip. From these data 1152 earthquakes are well-located within the network, and jointly inverted for hypocenters and 3D velocity structure. Beneath the continental Moho, defined by receiver functions, lies a 10 to 15 km thick gradient zone with Vp increasing from 7.0 to 7.9 km/s. This may reflect an underplated mafic, granulite-facies lowermost crust, or perhaps magmatic intrusion into lithospheric mantle. Otherwise, velocities in continental crust show typical continental values (Vp = 5.6 - 6.9 km/s), indicating that basaltic magmatism prior to the onset of seafloor spreading does not form a significant crustal addition. By contrast, we observe Vp = 6.5 - 7.2 km/s close to the oceanic rift tip in a zone 60 km wide, indicating a narrow region of mafic crust. This sharp contrast suggests that the transition from distributed continental rifting to localized seafloor spreading likely occurs across a narrow zone, close to the onset of spreadng in both time and space. Most earthquakes lie along a narrow belt extending from the oceanic rift tip westward along strike, in continental crust. This belt lies just north of the chain of young metamorphic core complexes and follows a graben structure identified on high-resolution bathymetry; focal mechanisms confirm that it represents a normal fault system. The fault system also follows the axis of maximum crustal thinning, and lies above a swath of very low upper-mantle velocities inferred to represent shallow asthenosphere. Thus, both brittle faulting and lithosphere-scale thinning are localized in an along-strike extension of the ridge axis, and it seems possible that this belt denotes the site of future seafloor spreading. Some seismicity in the continental crust follows other trends, occasionally following apparent transfer fault systems acting to broaden the region of deformation. It appears that some process localizes lithospheric thinning and faulting in the upper crust withing 100 km of the oceanic rift tip, as the continental crust completely extends before forming a new ocean basin.
DE: 8100 TECTONOPHYSICS
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005