Preliminary results from forward modelling the basement of the PreCaspian and Amadeus Basins give new insights into the generation of ultra deep basins
Deirdre Duggan, Paul Ryan
Geology Dept, NUI Galway, Galway, Ireland
The PreCaspian Basin of central Eurasia and the Amadeus Basin of central Australia have sediment thicknesses in excess of 20 km. The depth of the PreCaspian Basin, defined during the soviet Deep Seismic Sounding surveys begun in 1948, has been confirmed by the NARS-DEEP survey. The depth of the Amadeus Basin was defined during deep seismic reflection surveys by the Australian Geological Survey Organisation in 1985 and 1993. In both basins high resolution seismics have resolved the crustal structure and highlighted the major differences in crustal elements and sedimentary structures. The PreCaspian crust is characterised by a High Velocity Layer at mid-crustal depths, which could be either the lowest sediment layer, or the uppermost crustal layer. The Amadeus Basin crust is composed of a block of old, relatively undeformed crust wedged between two highly deformed orogenic belts.
A sedimentary basin of about 12 km deep cannot be accommodated by existing isostasy and extensional models. A numerical approach developed in NUI, Galway is used to investigate whether such sedimentary thicknesses can be created by a variation of the pure shear model of basin formation. Using a range of standard initial parameters (density variation within the crust=2600-3200 kgm-2, crustal thickness=25-55 km) and a range of deformation times, it will be shown that other factors must have come into play to generate these very deep basins. Conditions investigated are initial lithospheric structure, basaltic underplating and repeated rifting events not separated by inversion.