The Virtual Earth

One important activity is the construction of The Virtual Earth. The Virtual Earth describes the real or material Earth - it is an electronic rendition of our planet as it is today. The aspect upon which we have specifically focussed is the geometric description of the planetary lithosphere, in particular the 3D geometry of the major subducting slabs. These geometries have been derived by examination of hypocentric datasets in combinaiton with images derived from seismic tomography (e.g. Richards et al. 2005). The tomographic data has been analyzed in a way that emphasizes the median of any particular lithospheric slab. Errors of interpretation will abound, for seismic tomography is a science that is still in its infancy and ray-path density in many areas is too low to allow anything but subjective interpretation. Nevertheless, there are advantages to an overall assessment of possible geometry and the plethora of results that continue to emerge as the result of this analysis.

This 2D mesh is embeded in 3D space, and is intended to describe the geometry of the median plane of subducting lithosphere under South America. It was prepared by Dr Simon Richards (now at JCU) for the ACcESS MNRF. The Virtual Earth is made up of several such 3D meshes that describe the geometry of the planetary lithosphere in present subduction zones. This part of the Pplates Virtual Earth project involves ongoing collaboration between Professor Gordon Lister at ANU and Dr Simon Richards at JCU.

The Virtual Earth was initiated by Dr Simon Richards and Professor Gordon Lister. To contribute to its construction please contact one of us. Industry support is needed to allow The Virtual Earth to continue to the next stage of its development (universal grid-enabled access to the data as part of the AuScope NCRIS initiative).

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Papua New Guinea and the Solomon Islands

A Virtual Earth Project by Tomas O'Kane, PhD Candidate

3-D model of the Wadati-Benioff zone associated with the subducting Solomon Sea plate, developed by Tomas O'Kane under the supervision of Dr Simon Richards. This required the interpretation of the earthquake hypocentre dataset Engdahl et al., (1998), kindly provided by Robert Engdahl via personal communication. We have recently been examining the geodynamic implications of this inferred slab geometry.

The Papua New Guinea and Solomon Islands region is located within the transition zone between SE Asia and the SW Pacific, one of the best-endowed metallogenic belts and most actively deforming areas on Earth. Interactions between the WNW-moving Pacific Plate and the NNE-moving Australian Plate give rise to a complex convergent zone exhibiting numerous micro-plates, volcanic provinces, island arcs and some of the fastest relative plate motions on Earth.

Some of this complexity has been unraveled using earthquake hypocentre data to create a 3-D model of the Wadati-Benioff zone associated with the subduction of the Solomon Sea Plate at the New Britain Trench. We report the presence of a large aseismic zone, herein termed the New Ireland aseismic zone, located in the 'apex' of the subducting slab, the margins of which initially record a dip of 30 degrees, before steepening to over 75 degrees at depth. Steeper slab dips are seen both to the east and west of the slab apex, particularly the eastern section of the slab, which appears to have been overturned (i.e. dip exceeds 90 degrees).

In the light of this discovery, existing tectonic models must be reassessed. The 3-D geometry appears to require slab-tearing during roll-back, and by implication must have stranded formerly attached segments of the subduction zone. Alternative models for the aseismic zone, such as the subduction of an aseismic ridge, do not provide a satisfactory explanation for the apparent geometry (and the implied deformation) of the Solomon Sea slab. The roll-back and slab-tearing hypothesis is consistent with the current 3-D geometry and has significant ramifications for the tectonic evolution of the region.