Mechanism for Normal Faulting in the Subducting Plate at the Mariana Trench

Abstract:

We investigate the characteristics of normal faulting between the trench and outer rise in the subducting Pacific plate through analysis of high-resolution multi-beam bathymetry and geophysical data and geodynamic modeling. Analysis of multi-beam bathymetry data reveals significant variations in normal faulting characteristics along the Mariana trench: (1) The vast majority of the observed surface normal faulting scarps are observed to be sub-parallel to the local strike of the Mariana trench axis, indicating that the orientation of normal faults is predominantly controlled by subduction-related stresses rather than by pre-existing abyssal hill fabrics. (2) Trench-parallel normal fault scarps become apparant as the subducting plate approaches the outer rise of the Mariana trench, indicating that normal faulting initiates in this region. (3) Along the Mariana trench, the Challenger Deep region is associated with the greatest trench depth and largest average values of normal fault throw, while regions with seamounts near the trench axis show the smallest average values of fault throw. To explore the mechanisms that control normal faulting in a subducting plate, we perform numerical simulations of elasto-plastic plate subjected to tectonic loading, bending, and horizontal forces from slab pull. Modeling results suggest that bending-induced extensional stresses in the upper plate reaches maximum values near the outer rise, consistent with the onset of normal faulting in this region. However, bending alone does not predict the continued growth of normal faults toward the trench. We hypothesize that this additional fault growth could be related to (1) tectonic stresses induced by steep topographic slopes; and/or (2) slab pulling forces that are originated in the upper mantle due to the negative buoyancy of a subducted slab but are transmitted to the shallower part of the lithospheric plate prior to its subduction.