Petrophysical, geochemical, and hydrological evidence for extensive fracture-mediated fluid and heat transport in the Alpine Fault's hanging-wall damage zone

Abstract

© 2017. American Geophysical Union. All Rights Reserved. Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation.

3D Active Source Seismic Imaging of the Alpine Fault Zone and the Whataroa Glacial Valley in New Zealand

“…If so, the interpreted damage zone would then be nearly 600 m wide and exhibit reflectors with dip angles of 40°–56°. The dip angles of the imaged reflectors are well within the range of observed values from other analyses showing dips of 45°–65° both in the DFDP‐2 borehole (Massiot et al., 2018; Townend et al., 2017; Toy et al., 2017) and extrapolated shallow trenches (Langridge et al., 2018).…”

“…Thus, this fault or fracture zone might have a significant contrast in acoustic impedance in order to produce seismic reflections and to be visible in seismic images. However, it is not well constrained in other studies (Janku‐Capova et al., 2018; Jeppson & Tobin, 2020a; Massiot et al., 2018; Townend et al., 2017).…”

“…(2018) can attain a width of up to 1–10 km and reflects a superposition of various fractures that cause seismological and geodetic alteration. This broad damage zone is observed to be hydrogeological active (Janku‐Capova et al., 2018; Menzies et al., 2016; Townend et al., 2017). Extensive fracturing enables fluid flow and is observed to be both inherited from previous structures and fault‐related (Massiot et al., 2018; Simpson et al., 2020; Williams et al., 2018).…”

Seismic Anisotropy and Its Impact on Imaging the Shallow Alpine Fault: An Experimental and Modeling Perspective

“…Field observations are shown in colored boxes: Green is the sonic log wave speed recorded in the DFDP‐2B well from Townend et al. (2017), pink is the low‐velocity zone (LVZ) defined from teleseismic and refraction seismic analysis by Stern et al. (2007), and yellow is the LVZ defined by Feenstra et al.…”

“…We compare our experimental and numerically simulated wave speeds at low effective pressures to the DFDP‐2B borehole sonic log acquired in 2014 (Figure 11). The DFDP‐2B well drilled to a depth of 818 m beneath the Whataroa Valley in the Central Southern Alps of New Zealand as part of the Deep Fault Drilling Project (Massiot et al., 2018; Townend et al., 2017; Toy et al., 2017). The sonic log records wave speeds at a range of angles to foliation (normal to foliation to 50° from normal) due to the high deviation of the DFDP‐2B borehole.…”