The Argo Abyssal Plain : A Proposal to Drill Reflecting Interfaces within Oceanic Crust

The presence of dipping reflectors and zones of high reflectivity in oceanic crust are beingincreasingly recognised on modern seismic reflection data. Some workers have related thereflection characteristics of oceanic crust and its associated Moho to spreading rate - slow-spreading crust is proposed to have the richest variety of reflections, but with a poorly imagedMoho, where as fast-spreading crust is suggested to be largely transparent but with a strongreflection Moho. The various reflection characteristics of slow-spreading crust have beenassociated with both tectonic and/or magmatic origins, but whatever their causes, they areclearly important to understanding the accretion processes that occur at spreading ridges.Although imaging of the seismic characteristics and reflector geometries of oceanic crust hasimproved dramatically in recent years, without direct information on the nature of thereflecting zones it will not be possible to uniquely constrain their origins and to test competing models. Recent AGSO deep-seismic data over the Argo Abyssal Plain off northwest Australia provideexcellent imaging of both strong crustal reflections and an associated Moho from the world'soldest (Late Jurassic) preserved oceanic crust. This area represents an ideal location at whichto test the origin of a variety of oceanic crustal reflectors, and thus could make some majoradvancements in our understanding of spreading ridge accretion processes. The proposedsites, in conjunction with the nearby Site 765, will provide a unique insight into the nature,origin, variability and deformation of the oceanic crust in the region. Although the chilling willpush the capabilities of the Joides Resolution to the limit, this is likely to be the case for anysites targeting such objectives within oceanic crust. The main general objectives of the proposal are to: 1.Test models for the formation (tectonic and/or magmatic) of major crustal reflectionfeatures and zones in oceanic crust, and determine their significance to crustal accretionprocesses at spreading ridges; 2.Determine the cause of the reflectivity of the upper part of oceanic basement and itsimplications for the composition and physical properties of the crust; 3. Examine the thermal and mechanical evolution of oceanic crust as it ages and moves awayfrom the spreading ridge, and, in particular, the nature and cause of any late-stagereactivation of primary structures.