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Abstract The dating of volcanic tephras forms a critical cornerstone of chronostratigraphy and is paramount for the resolution of the geological timescale. (U‐Th[‐Sm])/He dating is an emerging tool in Quaternary tephrochronology and ideally suited to date tephras <1 Ma. We present zircon, magnetite and apatite (U‐Th[‐Sm])/He combined with zircon U‐Pb data for a Pleistocene tephra in syn‐rift strata of the Woodlark Rift in Papua New Guinea. The results reveal a young He age mode (~0.5 to 0.8 Ma), consistent with an autocrystic zircon U‐Pb crystallisation age of 0.8 ± 0.1 Ma, as well as a broad range of older (U‐Th[‐Sm])/He (~1.6 to 10.2 Ma) and U‐Pb (~4.4 to 107 Ma) ages. These data demonstrate the potential of integrated U‐Pb and (U‐Th[‐Sm])/He multi‐method chronometry for dating the youngest coherent age mode, detecting contaminant grains and evaluating the isotopic systematics of these techniques.
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Abstract We use densely spaced campaign GPS observations and laboratory friction experiments on fault rocks from one of the world's most rapidly slipping low‐angle normal faults, the Mai'iu fault in Papua New Guinea, to investigate the nature of interseismic deformation on active low‐angle normal faults. GPS velocities reveal 8.3 ± 1.2 mm/year of horizontal extension across the Mai'iu fault, and are fit well by dislocation models with shallow fault locking (above 2 km depth), or by deeper locking (from ~5–16 km depth) together with shallower creep. Laboratory friction experiments show that gouges from the shallowest portion of the fault zone are predominantly weak and velocity‐strengthening, while fault rocks deformed at greater depths are stronger and velocity‐weakening. Evaluating the geodetic and friction results together with geophysical and microstructural evidence for mixed‐mode seismic and aseismic slip at depth, we find that the Mai'iu fault is most likely strongly locked at depths of ~5–16 km and creeping updip and downdip of this region. Our results suggest that the Mai'iu fault and other active low‐angle normal faults can slip in large (Mw > 7) earthquakes despite near‐surface interseismic creep on frictionally stable clay‐rich gouges.
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Abstract Inherited structural, compositional, thermal, and mechanical properties from previous tectonic phases can affect the deformation style of lithosphere entering a new stage of the Wilson cycle. When continental crust jams a subduction zone, the transition from subduction to extension can occur rapidly, as is the case following slab breakoff of the leading subducted oceanic slab. This study explores the extent to which geometric and physical properties of the subduction phase affect the subsequent deformation style and surface morphology of post subduction extensional systems. We focus on regions that transition rapidly from subduction to extension, retaining lithospheric heterogeneities and cold thermal structure inherited from subduction. We present numerical models suggesting that following failed subduction of continental crust (with or without slab breakoff), the extensional deformation style depends on the strength and dip of the preexisting subduction thrust. Our models predict three distinct extensional modes based on these inherited properties: (1) reactivation of the subduction thrust and development of a rolling‐hinge detachment that exhumes deep crustal material in a domal structure prior to onset of an asymmetric rift; (2) partial reactivation of a low‐angle subduction thrust, which is eventually abandoned as high‐angle, “domino”‐style normal faults cut and extend the crust above the inherited thrust; and (3) no reactivation of the subduction fault but instead localized rifting above the previous subduction margin as new rift‐bounding, high‐angle normal faults form. We propose that the first mode is well exemplified by the young, rapidly exhumed Dayman‐Suckling metamorphic core complex that is exhuming today in Papua New Guinea.