Regional pore-fluid pressures in the active western Taiwan thrust belt: A test of the classic Hubbert-Rubey fault-weakening hypothesis

We document regional pore-fluid pressures in the active Taiwan thrust belt using 55 deep boreholes to test the classic Hubbert-Rubey hypothesis that high static fluid pressures (depth normalized as λ = Pf/ρrgz) account for the extreme weakness of thrust faults, since effective friction μf∗=μf(1−λ). Taiwan fluid pressures are dominated by disequilibrium compaction, showing fully compacted sediments with hydrostatic fluid pressures at shallow depths until the fluid-retention depth zFRD ≈ 3 km, below which sediments are increasingly undercompacted and overpressured. The Hubbert-Rubey fault weakening coefficient is a simple function of depth (1 − λ) ≈ 0.6zFRD/z. We map present-day and pre-erosion fluid pressures and weakening (1 − λ) regionally and show that active thrusts are too shallow relative to zFRD for the classic Hubbert-Rubey mechanism to be important, which requires z ≥ ∼4zFRD ≈ 12 km to have the required order-of-magnitude Hubbert-Rubey fault weakening of (1 − λ) ≤ ∼0.15. The best-characterized thrust is the Chelungpu fault that slipped in the 1999 (Mw = 7.6) Chi-Chi earthquake, which has a low effective friction μf∗≈0.08-0.12, yet lies near the base of the hydrostatic zone at depths of 1-5 km with a modest Hubbert-Rubey weakening of (1 − λ) ≈ 0.4−0.6. Overpressured Miocene and Oligocene detachments at 5-7 km depth have (1 − λ) ≈ 0.3. Therefore, other mechanisms of fault weakening are required, such as the dynamical mechanisms documented for the Chi-Chi earthquake.