How continents break up: Insights from Papua New Guinea

Abstract

Abstract. The Woodlark Basin in the western Pacific forms a continuous system of active continental rifting evolving to well-developed seafloor spreading. Thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin. Magnetic isochrons indicate a single Euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins. In agreement with rigid plate tectonic models, continental rifting initiated geologically synchronously (at --6 Ma) along the length of the protomargins within a deforming plate boundary zone. Strain localization and seafloor spreading, however, developed in a time transgressive fashion from east to west within this zone of deformation. Spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the Solomon and Coral Seas. The transition to spreading occurred after a rather uniform degree of continental extension: 200+_40 km. Both early and late stage rifting involved highand low-angle normal faults. We identify distinct styles in the transition from rifting to spreading which we refer to as nucleation, propagation, and stalling. These breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures. Continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating. The initial spreading segments achieved much of their length at nucleation (within rift basins separated along strike by accommodation zones), with subsequent lengthening by spreading propagation into rifting continental crust. This early propagation, and the subsequent development of transform faults between initially nontransform spreading segment offsets, produced rift and spreading segmentation boundaries that are not simply correlated. The spreading centers nucleated approximately orthogonal in strike to the opening direction but, as the protomargins were oblique to this direction, nucleation jumps occurred in order to maintain the new spreading centers within the protomargins. Thus stepwise spreading nucleation in order to remain within a rheologically weak zone, rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis, is the dominant form of the rifting-to-spreading transition in the Woodlark Basin.

High-angle, not low-angle, normal faults dominate early rift extension in the Corinth Rift, central Greece

“…The elimination of the Corinth Rift as an example of a rift that deforms predominantly by low-angle normal faulting brings into question the importance of LANFs at early rift zones generally. LANFs are observed in continental rifts that (1) have undergone significant stretching and are close to breakup (e.g., Woodlark Rift, Papua New Guinea; Taylor et al 1999), (2) exhibit significant magmatic activity or base-crustal shear stress to rotate stress tensors (e.g., Basin and Range and central Apennines), or (3) have preexisting thrust faults optimally orientated for reactivation (e.g., central Apennines). The Corinth Rift is an example of a mildly stretched rift, which is amagmatic, and normal faulting is perpendicular to the Hellenide orogeny; thus none of these conditions apply.…”

The Afar volcanic province within the East African Rift System: introduction

“…Dunbar & Sawyer (1989) use numerical models to demonstrate the role of zones of pre-existing lithosphetic weakness in the localization of strain at break-up, with or without the development of lithospheric-scale detachments. Taylor et al (1999) andHebert et al (2001) document the along-axis propagation of oceanic tiffing into the continental lithosphere through the emplacement of discrete mantle upwellings and ridge jumps, while Kusznir & Tymms (2003) have recently introduced models of depth-dependent stretching in which upper crustal extension is much less than that of the lower crust and lithospheric mantle, to…”

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

“…The detachment concept is now widely applied in extensional and passive margin settings (e.g. Froitzheim & Eberli 1990;Lister et al 1991;Reston et al 1996;Driscoll & Karner 1998;Hodges et al 1998;Osmundsen et al 1998;Taylor et al 1999;Boncio et al 2000;Manatschal et al 2001;Canales et al 2004). However, the apparent conflict between generally accepted geological interpretations and rock mechanical and seismological considerations has yet to be resolved satisfactorily (Sibson 1985;Jackson 1987;Jackson & White 1989;Collettini & Sibson 2001;Scholz & Hanks 2004;cf.…”