Variations in normal faulting and plate deformation along the Mariana trench

Abstract:

We investigate variations in normal faulting along the Mariana trench through analyzing deformation of the subducting Pacific plate and geodynamic modeling. Detailed investigation was focused on three areas where high-resolution multibeam bathymetry data are available: the Southern (S) region at 140.8–144°E; the Central (C) region at 14.7–16.8°N; and the Northern (N) region at 18–21.2°N. In each of the study regions, we analyzed seafloor normal faults and calculated the statistics of fault parameters including strike, throw, density, and location relative to the trench axis. Our analysis revealed several key constraints: (1) Most of the normal faults are sub-parallel to the local strike of the trench axis, indicating that the normal faults were initiated by bending stresses in the subducting plate and not by pre-existing seafloor abyssal tectonic fabrics. (2) The normal faults were likely initiated within the outer rise region and reached maximum throw toward the trench. (3) The S-region has the largest trench relief (average of ~6 km) as well as the largest fault throw (maximum throw reached ~320 m when averaged over 5 km of trench-perpendicular profiles). In contrast, the N-region has much smaller trench relief (~2 km) and smaller fault throw (maximum fault throw reached only ~180 m). We then modeled the subducting plate as an elasto-plastic slab subjected to tectonic forcing at the trench axis, including vertical load (V₀), bending moment (M₀), and horizontal tensional force (F₀). Using the above observations as constraints, modeling revealed the following key results: (1) The best-fitting solutions for the S-region are V₀ = 5.8 x 10¹² N/m, M₀ = 9.6 x 10¹⁶ N, and F₀ = 3.6 x 10¹² N/m, yielding the horizontal extensional force to vertical loading ratio F₀/V₀ = 62%. (2) The best-fitting V₀ for the N- and C-regions are about 33% and 41% that of the S-region. Meanwhile, the best-fitting M₀ for the N- and C-regions are about 80% and 60% that of the S-region. (3) The best-fitting F₀ for the N- and C-regions are calculated to be ~66% larger than that of the S-region. We hypothesize that the significant variations in normal faulting along the Mariana trench are related to variations in tectonic loading and variables including plate subduction geometry, plate properties, and presence of seamounts in the sub ducting plate.