Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture‐Mediated Fluid and Heat Transport in the Alpine Fault's Hanging‐Wall Damage Zone

Abstract

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.

Fluid Flux in Fractured Rock of the Alpine Fault Hanging‐Wall Determined from Temperature Logs in the DFDP‐2B Borehole, New Zealand

“…Some of the temperature anomalies with largest amplitude (Figure 8) coincide with anomalies in other observations, such as changes in geothermal gradient of the equilibrated temperature profile, increases of resistivity and natural gamma activity, a zone of drilling mud loss at $730 m depth Townend et al, 2017), or an increase in sonic amplitude and natural gamma activity at $470 m depth Figure 13. We compare the absolute values of amplitudes observed at different times after the last circulation in two depth regions and compare them with models of fluid flux (5 3 10 27 m s 21 ) and thermal diffusivity contrast of 0.3 3 10 26 m 2 s 21 at the center of this depth range.…”

The Alpine Fault Hangingwall Viewed From Within: Structural Analysis of Ultrasonic Image Logs in the DFDP‐2B Borehole, New Zealand

“…Indeed, the orientation of FS1 features is constant throughout the logged interval, ruling out a significant error of the caliper log (and hence in the calculation of feature orientations); both FS2 and FS3 are present throughout the logged interval; and the change of proportion of FS2 and FS3 occurs within a zone of moderate to good image quality. The top of this transition zone at 470-480 m is also conspicuous in other wireline logs Townend et al, 2017). Resistivity values are lower, the full-waveform sonic log shows a lower amplitude, the gamma-ray values are lower in the 330-475 m interval than in the 475-888 m interval (Figure 3), and there is a large temperature anomaly at 478 m (Janku-Capova et al, 2018).…”

Variations in Seismogenic Thickness Along the Central Alpine Fault, New Zealand, Revealed by a Decade's Relocated Microseismicity

“…Figure 5 shows the final relocated seismicity distribution for 7,719 earthquakes from late 2008 to early 2017. Earthquake epicenters delineate the surface trace of the Alpine Fault and otherwise mainly occur on the southeastern side of the Fault (hanging wall), which is known to be highly fractured (Cox et al, 2015;Townend et al, 2017). We divide the hanging wall seismicity into three clusters according to the earthquakes' spatial distribution and magnitudes ( Figure 5).…”

“…DFDP has to date involved two stages, and its main goal was to drill into the Alpine Fault and examine the physics of faulting and seismogenesis inside the fault zone. Borehole measurements and observations revealed an active hydrothermal circulation system of high temperatures, high permeability, and elevated pore fluid pressure beneath the Whataroa Valley (Sutherland et al, ; Townend et al, ). These extreme hydrothermal conditions are inferred to exist due to heat advection from depth and topographically driven fluid flow through fractured rocks that concentrates heat within the valleys (Sutherland et al, ).…”