Shallow dips of normal faults during rapid extension: Earthquakes in the Woodlark‐D'Entrecasteaux rift system, Papua New Guinea

Abstract

Abstract. Earthquakes in the Woodlark-D'Entrecasteaux rift system reveal changes in fault geometry along strike, as extension progresses from continental to oceanic modes. In one segment, evidence is found for earthquakes on shallow-dipping normal faults. To examine these trends, locations of 147 earthquakes are redetermined, and focal mechanisms for 21 events are derived from waveform inversion. Where seafloor spreading occurs, east of 152øE, most earthquakes illuminate a NE-SW strike-slip fault north of the oceanic rift system along the Woodlark Rise. Elsewhere, earthquakes occur along a well-defined rift zone, and focal mechanisms reveal normal faulting with north-south T axes and depths less than 10 km. Most normal-faulting earthquakes show high-angle fault planes (dips 40ø-50ø), but several are consistent with slip on moderately low-angle planes dipping 15ø-35 ø. These low-angle nodal planes are only found in a region between 150.5øE and 152.5øE, where the transition occurs from seafloor spreading to continental rifting. A seismic reflection profile near 151 øE reveals a major normal fault that dips to the north at 25 ø -35 ø just landward of the oceanic rift tip. That fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied. Hence normal faults dipping -30 ø are likely to be seismically active in at least one part of this rift system. In this region just landward from the rift tip the highest extension rates are expected (--40 mm/yr) and strain appears localized on a single system of normal faults. Because shallow dips for normal faults are seen near the rift tip, but not elsewhere along strike, high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips. Perhaps high shear traction at the base of the upper crust, localized by nearby spreading, sufficiently alters the local stress regime.

Rifting and shallow-dipping detachments, clues from the Corinth Rift and the Aegean

“…Low-angle normal faults have been described in different geological situations including post-orogenic extension, exhumation of HP-LT metamorphic rocks and even intraoceanic rifting (Wernicke, 1981;1995;Lister et al, 1984;Avigad and Garfunkel, 1989;Abers et al, 1997;Axen et al, 1999;Hayman et al, 2003;Collettini and Holdsworth, 2004;Garcès and Gee, 2007). Such low-angle normal faults are often referred to as detachments.…”

Geologic, structural, and thermochronologic constraints on the tectonic evolution of the Sierra Mazatán core complex, Sonora, Mexico: New insights into metamorphic core complex formation

“…If true, these faults violate traditional Andersonian fault mechanics [Anderson, 1951] and imply that many of our fundamental assumptions on the state of stress and strength of the crust are incorrect [e.g., Wernicke, 1995]. In addition, seismologic observations show that low-angle normal faults (<30°dips) are not currently active [Jackson, 1987;Jackson and White, 1989;Collettini and Sibson, 2001] although Abers [1991] and Abers et al [1997] suggested that several large earthquakes occurred on lowangle normal faults. As a result, low-angle normal faults are often considered to be a unique mode of crustal extension that do not conform to traditional rock mechanics and are not active in modern extensional settings [Wernicke and Burchfiel, 1982].…”

Woodlark Basin

“…Abers (1991) and Abers et al (1997) determined source parameters and relocated earthquakes in the rifting region. The focal mechanisms are all extensional or strike-slip with northerly tension axes (T-axes; Fig.…”

“…Earthquake source parameters and seismic reflection data indicate that low-angle normal faulting is active in the region of incipient continental separation (Figs. 2-5; Abers, 1991;Taylor et al, 1995Taylor et al, , 1996Mutter et al, 1996;Abers et al, 1997). Leg 180 will drill a transect of sites (just ahead of the spreading tip) above, below, and through a low-angle normal fault to determine the vertical motion and horizontal extension history prior to seafloor spreading and to characterize the composition and in situ physical properties of the active fault zone.…”