International Geologiical Congress - Oslo 2008

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STT-07 Three-dimensional aspects of subduction zone processes: Insight from dynamic modelling, tectonic reconstructions and obse

 

Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism

 

Gideon Rosenbaum, The University of Queensland (Australia)
Massimo Gasparon, The University of Queensland (Australia)
Francesco P. Lucente, Istituto Nazionale di Geofisica e Vulcanologia (Italy)
Angelo Peccerillo, UniversitÓ degli Studi di Perugia (Italy)
Meghan S. Miller, Rice University (United States)
 

 

Tectonic activity in convergent plate boundaries commonly involves backward migration (rollback) of narrow subducting slabs and segmentation of subduction zones through slab tearing. In this study, we combined seismic tomography data and kinematic reconstructions to obtain a deeper insight into the complex geodynamics of Italian magmatism. Seismic tomography results show gaps within the subducting lithosphere, which are interpreted as deep (100-500 km) sub-vertical tear faults. The development of such tear faults is consistent with proposed kinematic reconstructions, in which different rates of subduction rollback affected different parts of the subduction zone.

Our model shows that tear faulting and slab breakoff played a fundamental role in controlling the temporal and spatial distribution of young (<10 Ma) magmatism in Italy and the Tyrrhenian Sea. We recognize that magmatic activity induced by tear faulting was a transitional phase between subduction-related arc magmatism and post-collisional magmas related to slab breakoff. This tectono-magmatic evolution marks the destruction of subduction zones, indicating the fate of subducting slabs following continental collision.

The magmatic signature associated with tear faulting is important because it can considerably enhance our ability to reconstruct modern and ancient convergent plate boundaries. Tear faults, unlike strike-slip faults that are associated with oblique collision or indentation, are characterized by magmatic activity induced by upwelling of asthenospheric material. Their geometry is attained by tearing and segmentation of lithospheric slabs during subduction rollback. In most retreating plate boundaries, this process is inevitable. This is because the subducting lithosphere is not uniform but typically consists of numerous seamounts, oceanic plateaus, aseismic ridges and continental fragments. Since these lithospheric anomalies are relatively buoyant, their arrival at the subduction zone is likely to impede subduction rollback, thus dividing the subduction zone into segments of different rollback velocities.

We conclude that lithospheric-scale tear faults play a fundamental role in the destruction of subduction zones. As such, they should be incorporated into reconstructions of ancient convergent margins, where tear faults are possibly represented by continental lineaments linked with magmatism and mineralization.

 

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