Deformational Processes Accommodating Slip on an Active Low-Angle Normal Fault, Suckling-Dayman Metamorphic Core Complex, Papua New Guinea
Detachment faults that can be shown to have slipped at dips <30° in highly extended continental crust are referred to as “Low-Angle Normal Faults” (LANFs). Their apparent low-dip angle exposed at the Earth’s surface contradicts Anderson-Byerlee frictional fault mechanics theory, which predicts that normal faults initiate at dips of ~60–70° and frictionally "lock-up" at dips <30–45°. LANFs also lack significant associated seismicity; yet, a small number of normal faults are demonstrably active today at low angles (<30°). To address the longstanding paradox of LANFs, this thesis focuses on one of the few known, and probably best-preserved, active LANFs on Earth: the Mai’iu fault in SE Papua New Guinea—a structure that bounds the topographic massif of the Suckling-Dayman Metamorphic Core Complex (SDM). In particular, three fundamental questions are investigated regarding LANFs by studying the deformational processes accommodating slip on the Mai’iu fault: (1) "How do continental detachment faults achieve low dips at the Earth's surface: do they originate as low-angle normal faults, or does the footwall of an originally steeper fault deform in response to a ‘rolling-hinge’ unloading process?”; (2) "What micro-scale deformation mechanisms accommodate slip on LANFs in a metabasaltic protolith, and how do they vary with depth—are LANFs prone to aseismic creep, or earthquakes, or both?"; and (3) "What stresses (principal stress orientations, differential stresses, stress ratios) drive slip on an active LANF, and how do they vary with depth?".
Structural field data and geomorphic data interpreted from GeoSAR-derived digital terrain models (gridded at 5–30 m spacing) and aerial photographs show that dip-slip on the active Mai’iu fault has exhumed a little-eroded, strongly corrugated, continuous fault surface on its footwall that is >28 km wide. The Mai’iu fault emerges from the ground at the range front near sea level with a northward dip of ~21°N (locally as low as ~15°) and flattens southwards over the ~3 km-high crest of the domal SDM. The southernmost mapped portion dips ~12°S. Uplift and exhumation of the footwall was accompanied by progressive back-warping of both the exhumed fault surface and an underlying foliation through >26° about an axis parallel to the fault strike. Antithetic-sense (i.e., northside-up) dip-slip motion on a widespread set of faults that cut the exposed footwall of the Mai’iu fault, and strike parallel to it, accommodated some of the inelastic footwall bending that caused the Mai’iu fault to develop a domal shape. In agreement with this rolling hinge-style bending, bedding-fault cutoff angles in a stranded rider block of former hangingwall rocks indicate that the Mai’iu fault had an initial surface dip of ≥40°. Today, aligned microseismicity 12–25 km downdip of the Mai'iu fault trace delineates a convex upward fault zone that steepens downward to a 30–40° dip.
Detailed microstructural, textural and geochemical data combined with chlorite-based geothermometry of fault rocks rapidly exhumed from 20–25 km depth (T~425°C) reveal the processes operating inside the Mai’iu fault zone. Deformation in mylonitic rocks in the footwall of the Mai’iu fault is controlled by sliding and rotation of a pre-existing fine-grained (6–33 μm diameter) mafic assemblage together with syn-tectonic chlorite precipitation. This diffusion-accommodated grain-boundary sliding in the mylonites took place at >270–370°C. At shallower levels (T≥150–270°C), the mylonites are overprinted by an overlying <3 m thick zone of foliated cataclasite. Fluid-assisted mass transfer and metasomatic reactions created the foliated cataclasite fabric during inferred periods of aseismic creep. Pseudotachylites and ultracataclasites mutually cross-cut both the foliations and one another, recording repeated episodes of seismic slip. In these fault rocks, paleopiezometry based on calcite twinning yields peak differential stresses of ~140–185 MPa at inferred depths of 8–12 km. These differential stresses were high enough to drive continued slip on a ~35° dipping segment of the Mai’iu fault, and to cause new brittle yielding of strong mafic rocks in the exhuming footwall of that fault. In the uppermost crust (<8 km; T<150°C), where the Mai’iu fault dips shallowly and is most misoriented for slip, active fault rocks are clay-rich gouges containing abundant saponite, a frictionally weak mineral (μ<0.28).
In summary, the Mai’iu fault evolves by the "rolling-hinge" mechanism involving progressive flexural back-tilting during exhumation of an originally moderate dipping fault. The Mai’iu fault is frictionally weak near the surface, where the fault is poorly oriented; the fault is strongest at 8–12 km depth, where it both creeps and nucleates earthquakes. Locally high differential stresses drive this slip on a part of the fault that is not particulary misoriented. This research elucidates the mechanics of low-angle normal faulting in the best-exposed and fastest-slipping LANF on Earth.