Multi-timescale mechanics of an active low-angle normal fault
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Detachment faults dipping < 30° commonly accrue 10’s of kms of offset and accommodate a large portion of crustal extension in moderately-to-highly extended regions. Slip on these high-offset low-angle normal faults remains perplexing due to their apparent misorientation relative to Andersonian principal stress directions. Classic fault mechanical theory predicts that normal faults should frictionally lock up and become abandoned at dips < 30°, yet geologic, seismological, and geodetic evidence shows that some low-angle normal faults slip actively. Despite evidence for actively slipping low-angle normal faults, few large earthquakes have been recorded on these structures. The scarcity and low long-term slip rates of active low-angle normal faults make it difficult to determine whether these faults rupture in large earthquakes based solely on seismological or geodetic records. In this dissertation, multi-disciplinary studies of the world’s most rapidly slipping low-angle normal fault are integrated to better understand the structural and tectonic evolution of detachment faults as well as to determine whether these faults slip in large earthquakes or predominantly creep aseismically. Bounding the actively exhuming Dayman-Suckling metamorphic core complex, the Mai’iu fault in Papua New Guinea dips 16-24° at the surface and has been estimated to slip at dip-slip rates of 8.6 ± 1.0 mm/yr to 11.7 ± 3.5 mm/yr. Geodynamic models suggest that weak zones and thermomechanical heterogeneities inherited from a previous subduction phase may have facilitated the formation of this long-lived detachment fault system (Chapter 2). Models of seismic-cycle deformation governed by rate-and-state friction show that the spatial distribution of fault rock frictional stability parameters strongly controls whether low-angle normal faults creep aseismically, slip in periodic large earthquakes, or slip in a mix of episodic creep events and earthquakes (Chapter 3). Surveying and U/Th dating of emerged coral reef platforms along the Goodenough Bay coastline show that tectonic uplift is episodic and imply that this segment of the detachment system slips in infrequent (440 – 1520 year recurrence) large (Mw > 7.0) earthquakes (Chapter 4). Velocities from a newly installed network of densely spaced campaign GPS sites reveal horizontal extension rates of 8.3±1.2 mm/yr (~8-11 mm/yr dip-slip) on the Mai’iu fault (Chapter 5). Laboratory friction experiments on exhumed Mai’iu fault rocks showing depth-dependent transitions in frictional stability help constrain inversions of kinematic models of the GPS velocities indicating that the Mai’iu fault is more strongly locked at ~5-16 km depth and creeping interseismically above 5 km depth. This result suggests that large (Mw > 7.0) earthquakes nucleate downdip of the low-angle portion of the Mai’iu fault and can propagate to the surface along the shallowly-dipping segment and/or more steeply-dipping splay faults in its hanging wall. In contrast to previous studies suggesting that active low-angle normal faults predominantly creep aseismically, this work implies that the active Mai’iu low-angle normal fault slips in infrequent large earthquakes accompanied by some shallow interseismic creep.