Imaging continental breakup using teleseismic body waves: The Woodlark Rift, Papua New Guinea

Abstract

This study images the upper mantle beneath the D'Entrecasteax Islands, Papua New Guinea, providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers. Differential travel times from P and S-wave teleseisms recorded during the 2010-2011 CDPapua passive seismic experiment are used to invert for separate V P and V S velocity models of the continental rift. A low-velocity structure marks the E-W axis of the rift, correlating with the thinnest crust, high heat flow, and a linear trend of volcanoes. This slow region extends 250 km along strike from the oceanic spreading centers, demonstrating significant mantle extension ahead of seafloor breakup. The rift remains narrow to depth indicating localization of extension, perhaps as a result of mantle hydration. A high-V P structure at depths of 90-120 km beneath the north of the array is more than 6.5% faster than the rift axis and contains well-located intermediate depth earthquakes. These independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction, although the polarity of subduction cannot be resolved. This geometry is gravitationally unstable; downwelling or small-scale convection could have facilitated rifting and rapid lithospheric removal, although this may require a wet mantle to be realistic on the required time scales. The high-V structure agrees with the maximum P,T conditions recorded by young ultra-high pressure rocks exposed on the rift axis and may be implicated in their genesis.

Southeast Papuan crustal tectonics: Imaging extension and buoyancy of an active rift

“…It could represent a remnant slab dipping either north or south, or some other sort of instability; given the broader setting, it is likely that this is a remnant of earlier Tertiary subduction. Teleseismic imaging [ Eilon et al , ] shows that the earthquakes lie inside an anomalously fast body consistent with such low temperatures, supporting the inference of a cold and possibly damp body just north of the rift axis.…”

“…The lithospheric mantle may have been removed from below by some combination of thermal and chemical erosion from upward percolating melts [ Holtzman and Kendall , ]. Low submoho velocities (Figure d) [ Jin et al , ] and high V P / V S ratios beneath the D'Entrecasteaux Islands [ Eilon et al , ] indicate the presence of small degrees of upper mantle melts (≤1%) or hydration that could contribute to lithospheric degradation. Jin et al [] also suggest the potential impact of felsic compositional heterogeneity in explaining the low velocities at lithospheric depth.…”

“…The axis of this low‐velocity mantle continues without break from the oceanic rift tip westward beneath the MCCs. It is bound sharply to the north by a high‐velocity low‐temperature mantle structure at depths less than 100 km beneath the Trobriand Platform [ Eilon et al , ; Jin et al , ]. The mantle shows anisotropic mantle fabrics similar to a mid‐ocean ridge with spreading‐parallel fast directions [ Eilon et al , ]; anisotropic body wave tomography shows that the spreading‐parallel fast directions lie within hot, asthenospheric mantle (Z. Eilon et al, A joint inversion for shear velocity and anisotropy: The Woodlark Rift, Papua New Guinea, submitted to Geophysical Journal International , 2015).…”

Structural and Geomorphic Evidence for Rolling‐Hinge Style Deformation of an Active Continental Low‐Angle Normal Fault, SE Papua New Guinea

“…The tectonic regime in SE Papua New Guinea is governed by the oblique‐convergent motion of the Pacific Plate (PAC) relative to the Australian Plate (AUS) creating a complex array of microplates in the collision zone (Figure ; Tregoning et al, ; Wallace et al, , ). The Woodlark‐Solomon Sea microplate rotates counterclockwise relative to the Australian plate about a nearby Euler pole resulting in a north‐south opening of the Woodlark Basin with extension rates increasing to the east (Eilon et al, ; Taylor et al, ; Wallace et al, ). In the east, seafloor spreading has occurred at rates >20–60 mm/year since ~6 Ma, as recorded by magnetic anomalies in the Woodlark Basin (Taylor et al, ).…”

Eduction, extension, and exhumation of ultrahigh‐pressure rocks in metamorphic core complexes due to subduction initiation

“…Seismic surface wave and body wave tomographic studies, using newly collected data, show that the crust and lithosphere under the islands is thinner than is inferred for the Papuan Peninsula [ Abers et al ., ; Eilon et al ., ]. The ∼50 to 100 km wide zone of low velocities extends from the base of the crust down to at least 100 km depth [ Eilon et al ., ]. The thin crust under the islands implies that the high average elevation of the islands is likely supported by hot, low‐density mantle asthenosphere.…”