The boundary between continental rifting and sea-floor spreading in the Woodlark Basin, Papua New Guinea

Abstract

Seismic reflection, bathymetry, acoustic imagery and magnetic data are presented that encompass the boundary between rifting of the Papuan continent and westward propagating sea-floor spreading in the Woodlark Basin. West of the spreading tip, the southern margin is characterized by large fault blocks, which were tilted to the south on north-dipping normal faults during the current rifting phase, and graben further south where previous rifting failed. The northern margin is devoid of large offset normal faults and its subsidence from near sea level requires synrift flow of the lower crust. The margin asymmetry primarily reflects across-strike differences in the prerift geology, morphology and rheology. Horst and graben with 3 km relief below sea level occur adjacent to the spreading tip. Although heat flow data imply greater crustal thinning to the south of Moresby continental seamount, seismicity shows that deformation is currently focused on the Moresby normal fault that bounds its northern side. Based on logging results from ODP Leg 180 and semblance analysis, depth conversion of seismic reflection data show that Moresby Fault has a dip ≥29°, which is compatible with an earthquake slip plane of 30°–33°. Directly east of Moresby Seamount, the first spreading segment is identified on the basis of magnetic, bathymetry and sidescan data to comprise two ring dykes in the west and Cheshire Seamount in the east. A seismic reflection line crossing the western ring dyke shows an intrusive body that cuts Moresby Fault at depth. Currently, intrusive rocks have only reached the surface near the ring dyke, leaving flat-lying synrift sediments, sills (?) and surface lava flows overlying the main intrusive body. The boundary between continental rifting and sea-floor spreading in the Woodlark Basin is spatially abrupt, although the processes overlap in time. The progression from rifting to spreading is characterized by a decrease in sedimentation as the margins are progressively thinned, subside below sea level, are eroded less and trap sediments in proximal basins. The post-rift sedimentation near the continent–ocean boundary is hemipelagic and drapes breakup topography without a distinct breakup unconformity. Synrift sediments, deposited above the 8.4 Ma rift onset unconformity and prior to breakup, are characterized by rotated sections (with parallel or diverging reflectors) as well as by ponded sections that onlap bounding fault blocks where local extension has ceased. Splayed reflectors representing sediment deposition on rotating fault blocks do not characterize the full synrift section, but only those times and places where extension is localized.

Anisotropy beneath a highly extended continental rift

“…Taylor et al . [; Goodliffe and Taylor , ] used magnetic anomalies in the Woodlark Rift east of 152°E to describe the relative WLK‐AUS motion since ∼8.4 Ma as anticlockwise rotation about an Euler pole at (9.3°S, 147°E) at −4.022°/Myr. New seafloor has been produced diachronously at westward propagating spreading centers since 4–6 Ma and the DI lie just west of the rift tip (151.7°E), along the extrapolated line of the youngest spreading center.…”

Overview of tectonic settings related to the rifting and opening of Mesozoic ocean basins in the Eastern Tethys: Oman, Himalayas and Eastern Mediterranean regions

“…In this case, the spreading tip migrated through a broad rift zone, within crust that was previously deformed and thermally immature Goodliffe & Taylor 2007). The spreading tip migrated westwards, episodically over several million years, a timescale that might not be currently detectable if applicable to continental breakup in the Eastern Tethyan region.…”

“…The tectonic architecture of one conjugate Non-volcanic rifted margin setting has been explored, utilizing two legs of drilling on the Iberia margin and, so far, one leg of drilling on the Newfoundland margin (Tucholke et al 2004. Additional evidence has come from the drilling of a relatively small number of other settings, notably the Equatorial African transform margin (Mascle et al 1987), and from back-arc basins including the Western Mediterranean Tyrrhenian Sea (Kastens et al 1988) and the SW Pacific Woodlark Basin Goodliffe & Taylor 2007). Several other rifted margins were studied mainly using geophysical techniques and have been assigned, tentatively either to Non-volcanic or Volcanic-rifted margin types.…”

“…A back-arc setting is quite attractive in principal as it provides a viable mechanism to open peripheral rift basins. Such rifts might be comparable with, for example, the Woodlark back-arc basin, SW Pacific, which developed within a longlived active margin setting (see Goodliffe & Taylor 2007) or the Tyrrhenian Sea, Western Mediterranean Sea, which opened in a regional convergent setting (Kastens et al 1988). Even if valid, this would not preclude valuable insights on rift processes, as shown by studies of the Woodlark rift basin, which is located in a regional back-arc setting (Goodliffe & Taylor 2007).…”

“…A migrating spreading centre can be compared with the westward-stepping migration of the Woodlark spreading centre during Late MioceneQuaternary time (see Goodliffe & Taylor 2007). In this case, the spreading tip migrated through a broad rift zone, within crust that was previously deformed and thermally immature Goodliffe & Taylor 2007).…”