The Tasmantid Seamounts: A window into the structural inheritance of ocean floor fabric

Abstract:

The extinct Tasman Sea spreading centre, active from 84--53\,Ma, is intersected at a number of locations by the Tasmantid Seamount Chain. The chain, which extends for over 2000\,km off the east coast of Australia, progressively increases in age from south to north with ages ranging between 6\,Ma and $\sim$50\,Ma. While thick sediment ($\sim$1\,km) obscures much of the northern Tasman Sea basement, detailed morphological and geophysical analyses of the seamounts reveal a strong correlation between tectonic setting, seamount orientation, and volcanic structure, despite the $\geq20$\,Ma offset between spreading cessation and initial seamount emplacement.

Morphologically, structural inheritance is evidenced by the contrast between two volcanic styles: 1) the rugged, predominantly fissure-fed, fabrics characterizing seamounts emplaced at inside corners of spreading segment-transform intersections; and 2) the conical seamounts with summit craters and isolated dyke-fed flank cones that develop off-axis. Furthermore, volcanic fabrics align closely with the principal stress directions expected for a spreading ridge system in which strong mechanical coupling occurs across transform faults. This suggests that the lithosphere is dissected by numerous deep faults, allowing magma to be channelled away from the site of melting along pre-existing structural trends. The generally low effective elastic thickness, $T_e$, ($\leq$15 km) and lack of a plate age-$T_e$ relationship along the chain indicate that structural inheritance is also the major control on lithospheric strength near the extinct spreading centre. While the importance of structural inheritance in controlling magmatic behaviour is commonly acknowledged in continental settings, these results clearly demonstrate the need to also consider it in the oceanic realm.

The extinct Tasman Sea spreading centre, active from 84--53\,Ma, is intersected at a number of locations by the Tasmantid Seamount Chain. The chain, which extends for over 2000\,km off the east coast of Australia, progressively increases in age from south to north with ages ranging between 6\,Ma and $\sim$50\,Ma. While thick sediment ($\sim$1\,km) obscures much of the northern Tasman Sea basement, detailed morphological and geophysical analyses of the seamounts reveal a strong correlation between tectonic setting, seamount orientation, and volcanic structure, despite the $\geq20$\,Ma offset between spreading cessation and initial seamount emplacement.

Morphologically, structural inheritance is evidenced by the contrast between two volcanic styles: 1) the rugged, predominantly fissure-fed, fabrics characterizing seamounts emplaced at inside corners of spreading segment-transform intersections; and 2) the conical seamounts with summit craters and isolated dyke-fed flank cones that develop off-axis. Furthermore, volcanic fabrics align closely with the principal stress directions expected for a spreading ridge system in which strong mechanical coupling occurs across transform faults. This suggests that the lithosphere is dissected by numerous deep faults, allowing magma to be channelled away from the site of melting along pre-existing structural trends. The generally low effective elastic thickness, $T_e$, ($\leq$15 km) and lack of a plate age-$T_e$ relationship along the chain indicate that structural inheritance is also the major control on lithospheric strength near the extinct spreading centre. While the importance of structural inheritance in controlling magmatic behaviour is commonly acknowledged in continental settings, these results clearly demonstrate the need to also consider it in the oceanic realm.