Using Syntectonic Calcite Veins to Reconstruct the Strength Evolution of an Active Low‐Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

Abstract

Lithospheric or crustal strength is often expressed as the maximum differential stress that a rock can sustain before it fractures or flows. Most knowledge of brittle rock strength comes from laboratory experiments (e.g., Byerlee, 1978) and deep boreholes (e.g., Hickman & Zoback, 2004;Zoback & Harjes, 1997). These studies typically infer Coulomb frictional failure in the upper crust, in which the differential stress or shear stress (τ) is linearly related to the effective normal stress (σ e ) via a "Byerlee" coefficient of friction (μ = 0.6-0.85, Behr & Platt, 2014;Byerlee, 1978). Detachment faults-associated with regional extension-that may have formed and/or slipped as "low-angle normal faults" (<30°, LANFs), remain a controversial topic because Andersonian normal faults in Mohr-Coulomb materials with "Byerlee" frictional strength should form and slip at dips of 60-75° and frictionally lock-up at dips <30-45° (e.g.,

The Dynamics of Unlikely Slip: 3D Modeling of Low‐Angle Normal Fault Rupture at the Mai'iu Fault, Papua New Guinea

“…The setup of Model 2 is identical to the reference model except that initial stresses are calculated based on a σ 1 orientation plunging 15° SSW, now consistent with the paleopiezometrically inferred footwall stress orientations (Mizera et al., 2021) and optimally oriented for dip‐slip on a normal fault dipping 75°. With such backwards‐inclined principal stresses, LANFs should be even less well‐aligned for slip than they are under Andersonian conditions, as illustrated by the decreased ratio of initial shear stress over initial effective normal stress (Figure 4b).…”

“…We simulate ruptures under different prestress conditions (Table 1) including Andersonian extension (Figures 4a and 4d) and stress fields where the maximum principal stress is non‐vertical and plunges toward or away from the dip‐slip direction (Figures 4b and 4c). Paleostress inversions (Mizera et al., 2021) and ongoing folding of the megacorrugated footwall (Little et al., 2019; Webber et al., 2020) indicate that strike‐perpendicular extension is accompanied by strike‐parallel constriction. The reference model, model 1 (Figure 4a, Table 1), employs an Andersonian stress field with the intermediate principal stress σ 2 oriented along‐strike (N60°W) resulting in the minimum principal stress σ 3 aligned parallel to the extension direction (N30°E).…”

“…The relative magnitudes of the principal stresses can be quantified by the stress-shape ratio, Φ = 𝐴𝐴 𝐴𝐴2 − 𝐴𝐴3∕𝐴𝐴1 − 𝐴𝐴3 , where Φ ranges from 0 (uniaxial compression with σ 1 >> σ 2 𝐴𝐴 ≈ σ 3 ) to 1 (axial constriction with σ 1 𝐴𝐴 ≈ σ 2 >> σ 3 ). Following Mizera et al (2021) who report strongly constrictional stresses, we assume Φ = 0.8 in all models.…”

“…Failed nucleation in Model 2 implies that applied stresses on the Mai'iu fault are more similar to those in its hanging wall than its footwall, suggesting that effective stress on the seismogenic portions of detachments may be limited by the strength of the hanging wall, which is typically lower than that of the metamorphic footwall due to the presence of weaker, less consolidated, unmetamorphosed sedimentary rocks. Additionally, stresses in the footwall and hanging wall may be largely decoupled if the detachment is weak, as suggested by paleopiezometry and geodynamic models of the Mai'iu fault (Biemiller et al, 2019;Mizera et al, 2021), not unlike local stress orientation heterogeneities observed in the shallow subduction margins of Nankai (Lin et al, 2016) and Hikurangi (McNamara et al, 2021) which may be tied to effective stress and pore pressure discontinuities across the weak, impermeable shallow subduction interface fault zones (e.g., Skarbek & Saffer, 2009).…”

“…In contrast, Mizera et al. (2021) jointly inverted paleostress orientations recorded by calcite twins in the exhumed footwall of the Mai'iu fault along with the orientations of smaller faults in its footwall and hanging wall to infer principal stress orientations in both sections. Their analysis (Figure 1b) found that the hanging wall is characterized by extensional Andersonian stresses with a vertical maximum principal stress and a horizontal minimum principal stress aligned parallel to extension.…”