CONGESTED SUBDUCTION ZONES: INSIGHTS FROM EAST GONDWANA RECONSTRUCTIONS AND DYNAMIC MODELS

Global patterns of congested subduction show alternating patterns of trench advance and retreat, which drives different tectonic modes in the overriding plate. Trench advance occurs in the collision front, whereas regions adjacent to the collision front undergo extension due to subduction rollback. We explore this concept in context of collision of the Hikurangi Plateau on the eastern margin of Gondwana.

Since the Cretaceous the margin of east Gondwana has transformed from an Andean-style margin to a complex series of backarcs, triggering a reconfiguration of the plate boundaries in the Southern Pacific Ocean. We ask what triggered this switch? Using a combination of plate kinematic reconstructions, 3D geodynamic modeling, and geological and geophysical syntheses we investigate this question.

The age of the converging Pacific plate was young (<30 m.y.) along the Antarctic part of the Gondwana margin during the Cretaceous (Seton et al., 2012). This lithosphere was carrying the buoyant Hikurangi Plateau, which is interpreted to have collided at ~110-100 Ma causing localized re-organization of the plate margin (Mortimer et al., 2019). This collision congested subduction, suturing the Hikurangi Plateau with Gondwana. This collision triggered subduction rollback and extension in the overriding plate adjacent to the collision front. This extension is evident in the South Island of New Zealand. Metamorphic core complexes with ages of ~112-109 Ma (Forster and Lister, 2003), supporting the idea that extension was contemporaneous with collision.

Based on these observations we propose that collision between the Hikurangi Plateau and Gondwana occurred along the north Island of New Zealand. Geodynamic models performed in Underworld (Moresi et al., 2007) suggest an alternative way to merge the Hikurangi Plateau with the Chatham Rise via asymmetric subduction rollback, tearing the extending Chatham and Campbell plateaus from Gondwana and translating them against the Hikurangi Plateau during reconfiguration of the plate margin. We propose that this extension and rollback triggered the opening of the Tasman Sea and rifting of Zealandia from East Gondwana. This proposed model has significant implications for plate tectonic configurations of the South Pacific.