Diffuse Extension of the Southern Mariana Margin

Abstract

Extension within the southern Mariana margin occurs both normal to and parallel to the trench. Trench‐normal extension takes place along focused and broad backarc spreading axes forming crust that is passively accreted to the rigid Philippine Sea plate flank to the northwest. To the southeast, trench‐parallel extension has split apart the Eocene‐Miocene forearc terrain accreting new crust diffusely over a 150–200 km wide zone forming a pervasive volcano‐tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates active extension over this forearc region and basement samples date young although waning volcanic activity. Such diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that in a near‐trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid‐ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting.

Upper Plate Stress Controls the Distribution of Mariana Arc Volcanoes

“…Our study is consistent with the findings of Martínez et al () and Martinez et al (), who showed that lineaments display contrasting orientation on either side of the spreading axis at its southern end. From the spreading axis to the West Mariana Ridge, as described above, most lineaments are parallel to spreading axis orientations while from the spreading axis to the Mariana Ridge lineaments are normal to the spreading axis and the trench (oriented in ENE‐WSW and ESE‐WNW directions).…”

“…Therefore, we infer that the trenchperpendicular tensional stress in the south arc-segment formed in a similar stress regime to structural lineaments of the Southern Mariana seafloor and was mainly caused by rollback of the subducting Pacific Plate. Martinez et al (2018) have suggested that the absence of discrete central arc volcanoes here is due to the more diffuse focus of deep arc stress. This contrasts with the concentration of stress in the deep arc lithosphere between 13.5°and 23°N, which produces a more distinct arc front.…”

“…Light blue arrows indicate the minimum horizontal stress ( σ Hmin ) as the minimum principal stress ( σ 3 ) direction identified from the mapped lineament while dark blue arrows indicate the maximum horizontal stress ( σ Hmax ) as the maximum principal stress ( σ 1 ) direction. Principal lineament orientations imply that extension occurs normal to the trench with secondary extension parallel to the trench to accommodate the strong curvature of the margin (Martinez et al, ).…”

“…Light blue arrows indicate the minimum horizontal stress (σ Hmin ) as the minimum principal stress (σ 3 ) direction identified from the mapped lineament while dark blue arrows indicate the maximum horizontal stress (σ Hmax ) as the maximum principal stress (σ 1 ) direction. Principal lineament orientations imply that extension occurs normal to the trench with secondary extension parallel to the trench to accommodate the strong curvature of the margin (Martinez et al, 2018). in the south. These differences reflect a stronger pull-down force on the overriding plate and stronger, trench-perpendicular compressional force in the shallow parts of the upper plate in the north and midnorth compared to the south segment.…”

Deep Seismic Structure Across the Southernmost Mariana Trench: Implications for Arc Rifting and Plate Hydration

“…The degree of arc evolution is variable along the IBM subduction system. The Southwest Mariana Rift (SWMR) is located at the southernmost Mariana, which separates the Western Mariana Ridge (WMR) and forearc blocks (Martinez et al, ). The SWMR differs from the Mariana Trough (MT) along the central‐north Mariana.…”