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

Abstract

a b s t r a c tWe document regional pore-fluid pressures in the active Taiwan thrust belt using 55 deep boreholes to test the classic HubberteRubey hypothesis that high static fluid pressures (depth normalized as l ¼ P f / r r gz) account for the extreme weakness of thrust faults, since effective friction m * f ¼ m f ð1 À lÞ. Taiwan fluid pressures are dominated by disequilibrium compaction, showing fully compacted sediments with hydrostatic fluid pressures at shallow depths until the fluid-retention depth z FRD z 3 km, below which sediments are increasingly undercompacted and overpressured. The HubberteRubey fault weakening coefficient is a simple function of depth (1 À l) z 0.6z FRD /z. We map present-day and pre-erosion fluid pressures and weakening (1 À l) regionally and show that active thrusts are too shallow relative to z FRD for the classic HubberteRubey mechanism to be important, which requires z !~4z FRD z 12 km to have the required order-of-magnitude HubberteRubey fault weakening of (1 À l) ~0.15. The bestcharacterized thrust is the Chelungpu fault that slipped in the 1999 (M w ¼ 7.6) Chi-Chi earthquake, which has a low effective friction m * f z0:08e0:12, yet lies near the base of the hydrostatic zone at depths of 1e5 km with a modest HubberteRubey weakening of (1 À l) z 0.4À0.6. Overpressured Miocene and Oligocene detachments at 5e7 km depth have (1 À l) z 0.3. Therefore, other mechanisms of fault weakening are required, such as the dynamical mechanisms documented for the Chi-Chi earthquake.

Fluid overpressures and strength of the sedimentary upper crust

“…A similar fluid-pressure depth pattern is found throughout the active western Taiwan thrust belt with z FRD z 3e3.5 km (Fig. 3A) (Yue, 2007;Yue and Suppe, 2014), with the predicted strength approximately constant below the fluid-retention depth (Fig. 3B).…”

“…(4), (1Àl) z 0.6z FRD /z. This relationship has been applied to thrust and accretionary wedge mechanics by Yue and Suppe (2014). Here we summarize the basic roles of overpressure in mechanics of thrust belts, in light of the results of this paper.…”

“…Point data are shut-in pressure tests from permeable strata, and the solid curve is pressure in shale computed from sonic log (Suppe and Wittke, 1977;Yue, 2007;Yue and Suppe, 2014). …”

“…Fluid-retention depth z FRD is particularly important because it can be observed based on commonly available borehole data and on seismic velocity analysis in non-eroded siliciclastic sedimentary sections. In addition, fossil fluid-retention depths can be preserved in uplifted and eroded strata and identified by a variety of means (Magara, 1978;Yue and Suppe, 2014). Once we know the fluidretention depth we can estimate the Hubbert-Rubey weakening (1 À l) z 0.6z FRD /z everywhere within the overpressured zone (Eq.…”

Structure and Deformation History of the Rapidly Growing Tainan Anticline at the Deformation Front of the Taiwan Mountain Belt

“…Porosity decreases relatively rapidly with depth, from 44–46% in the near surface, in agreement with independent porosity measurements (Wang et al, ), to 2–3% at 3.5‐km depth (Figure ), illustrating the increase in vertical compaction due to sediment overburden. While drilling TN‐1 borehole (Figure ), the mud weight had to be significantly increased at ~3.0‐km depth (Huang et al, ), which indicates higher pore fluid pressure and likely corresponds to the fluid retention depth, which is typically ~3 km in western Taiwan (Yue & Suppe, ). Below this depth, sediments reached a maximum in compaction, so that incremental sedimentary or tectonic load is supported by pore fluid instead of compaction (Suppe, ).…”