Propagation of back‐arc extension into the arc lithosphere in the southern New Hebrides volcanic arc

Abstract

New geophysical data acquired during three expeditions of the R/V Southern Surveyor in the southern part of the North Fiji Basin allow us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone, where it bends eastward along the Hunter Ridge. Unlike the northern end of the Tonga subduction zone, on the other side of the North Fiji Basin, the 908 bend does not correspond to the transition from a subduction zone to a transform fault, but it is due to the progressive retreat of the New Hebrides trench. The subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the New Hebrides arc and toward the south of the Hunter Ridge, so that the direction of convergence remains everywhere orthogonal to the trench. In the back-arc domain, the active deformation is characterized by propagation of the back-arc spreading ridge into the Hunter volcanic arc. The N-S spreading axis propagates southward and penetrates in the arc, where it connects to a sinistral strike-slip zone via an oblique rift. The collision of the Loyalty Ridge with the New Hebrides arc, less than two million years ago, likely initiated this deformation pattern and the fragmentation of the upper plate. In this particular geodynamic setting, with an oceanic lithosphere subducting beneath a highly sheared volcanic arc, a wide range of primitive subduction-related magmas has been produced including adakites, island arc tholeiites, back-arc basin basalts, and medium-K subduction-related lavas.

Diffuse Extension of the Southern Mariana Margin

“…The diffusely distributed deformation of the southern Mariana margin is distinct from other near-trench settings, such as at the northeastern end of the Tonga Trench (Wright et al, 2000) and southern end of the New Hebrides trench (Patriat et al, 2015), where well-organized (narrow plate boundary zone) spreading centers intersect the trench and take up trench-parallel extension. In these settings, the divergent axes are located behind the arc volcanic front and there is little to no slab underlying the spreading centers, which may affect the degree of hydration of the lithosphere and therefore lead to a stronger lithosphere and more rigid platelike behavior.…”

“…Journal of Geophysical Research: Solid Earth 10.1002/2017JB014684 the trench (Patriat et al, 2015;Pelletier et al, 1998). The curving Mariana and New Hebrides trenches imply some oblique convergence as the larger subducting plates have fairly uniform directions of motion across these trenches.…”

“…In most oceanic extensional settings, a period of tectonic rifting is followed by a focusing of extension leading to magmatic crustal accretion within a narrow plate boundary zone (seafloor spreading). Typical examples occur at STEP faults (Govers & Wortel, 2005) Ridge (Livermore, 2003), and the southern New Hebrides trench (Patriat et al, 2015). When oceanic rifts propagate into older lithosphere, the tectonic rifting phase may be more extensive than in young lithosphere, but once magmatic accretion occurs it tends to rapidly focus to a narrow axis Naar et al, 1991).…”

“…The curving Mariana and New Hebrides trenches imply some oblique convergence as the larger subducting plates have fairly uniform directions of motion across these trenches. In the New Hebrides system, strike-slip faults or zones develop inboard of the trench and take up the strike-slip component so that convergence and p-axis azimuths remain normal to the trench (Patriat et al, 2015). In the southern Mariana Trench, the strike-slip component is apparently taken up at the trench and subduction interface itself, as no strike-slip faults and very few strike-slip focal mechanisms are observed in the upper plate.…”

“…Another setting where limited hydration may affect plate boundary behavior is within backarc basin interiors distal from the trench. For example, in the North Fiji Basin, the curving New Hebrides trench leads to diverging rollback directions (Patriat et al, 2015) but narrow plate boundary zone triple junctions form in the plate interior (Auzende et al, 1994;Lafoy et al, 1990) (Figure 8b).…”