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

Abstract

Southeast Papua hosts the world's youngest ultra‐high‐pressure (UHP) metamorphic rocks. These rocks are found in an extensional setting in metamorphic core complexes. Competing theories of extensional shear zones or diapiric upwelling have been suggested as driving their exhumation. To test these theories, we analyze the CDPAPUA temporary array of 31 land and 8 seafloor broadband seismographs. Seismicity shows that deformation is being actively accommodated on the core complex bounding faults, offset by transfer structures in a manner consistent with overall north‐south extension rather than radial deformation. Rayleigh wave dispersion curves are jointly inverted with receiver functions for crustal velocity structure. They show crustal thinning beneath the core complexes of 30–50% and very low shear velocities at all depths beneath the core complexes. On the rift flanks velocities resemble those of normal continents and increase steadily with depth. There is no evidence for velocity inversions that would indicate that a major density inversion exists to drive crustal diapirs. Also, low‐density melt seems minor within the crust. Together with the extension patterns apparent in seismicity, these data favor an extensional origin for the core complexes and limit the role of diapirism as a secondary exhumation mechanism, although deeper mantle diapirs may be undetected. A small number of intermediate‐depth earthquakes, up to 120 km deep, are identified for the first time just northeast of the D'Entrecasteaux Islands. They occur at depths similar to those recorded by UHP rocks and similar temperatures, indicating that the modern seismicity occurs at the setting that generates UHP metamorphism.

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…At 550 °C and 0.8 GPa, they show that Vs lies near 3.8 km/s, and never below 3.7 km/s. While temperature variations are poorly known for these compositions, owing to the competing effects of thermal expansion and dehydration‐mediated reactions, most direct calculations on dry felsic rocks are unable to produce Vs < 3.6 km/s (e.g., Abers et al, ; Brownlee et al, ). Across the United States, at 25‐km depth Vs is less than 3.6 km/s in only 3.4% of the area resolved by Shen and Ritzwoller (), largely beneath volcanic areas of the Cascades or Basin and Range.…”

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

“…These so called low‐angle normal faults (LANFs) or detachment faults are typically domal in shape, and commonly separate tilted and faulted upper crustal unmetamorphosed rocks in the hangingwall from middle‐to‐lower crustal metamorphic rocks in the footwall (e.g., Armstrong, ; Coney, ; Platt et al, ; Tucholke et al, ). Activity and especially initiation of normal faults at such low dips contradicts the predictions of Coulomb failure (e.g., Anderson, ) and is not consistent with the observed seismicity in active rifts, which is dominated by slip on moderate to steeply dipping normal faults (Abers et al, ; Collettini & Sibson, ; Jackson, ; Jackson & White, ). Using paleomagnetic techniques, oceanic LANFs with slip magnitudes of tens of kilometers have been shown to have originated at dips of 45–60° (Garcés & Gee, ; Morris et al, ).…”

“…West of Goodenough Island, most of the Woodlark Rift extension is accommodated by the Mai'iu fault, a normal fault that has geologically and geodetically determined dip slip rates of ~12 mm/year (Wallace et al, ; Webber et al, ). Downdip of its surface trace, microearthquake foci scattered between 12 and 25 km depth (middle to lower crust) define a deformation zone that dips north at 30–40° (Figures b1 and b2; Abers et al, ). Abers et al () interpreted this deformation zone as the subsurface continuation of the Mai'iu fault.…”

“…Downdip of its surface trace, microearthquake foci scattered between 12 and 25 km depth (middle to lower crust) define a deformation zone that dips north at 30–40° (Figures b1 and b2; Abers et al, ). Abers et al () interpreted this deformation zone as the subsurface continuation of the Mai'iu fault. Previous workers have suggested that the Mai'iu fault formed by extensional reactivation of the older Owen‐Stanley Fault Zone (OSFZ), a Late Cretaceous‐Paleocene subduction thrust with an ophiolite in its hangingwall (Daczko et al, ; Davies, ; Little et al, ; Webb et al, ).…”

“…(a) Plate tectonic setting of the Woodlark Rift and present‐day GPS‐derived pole of rotation with the SDM highlighted in green (modified after Fitz & Mann, ; Wallace et al, ). (b1) Microseismicity map for part of the Woodlark Rift (acquired for the period March 2010 to January 2011 and modified after Abers et al, ). Dots: Color‐coded hypocenters of microearthquakes (depth scale bar in b2); red stars: Volcanoes.…”