Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

<p>During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.</p

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

International audienceFault 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

New grain growth experiments in water ice

Grain growth exponents are constrained independently of the thermally activated kinetics of growth. Two homogeneous samples with dissimilar grain sizes were subjected to a single heating event. The exponential n is proportional to the ratio of their change in grain size (D) (see thesis for equation). Heat treatment experiments were performed in superfine (< 25um) ice samples of variable porosity. A new type of superfine ice was synthesised. Spherical ice particles (2-30um) were generated by using an inkjet piezoelectric printer to spray micro-droplets of deionised water into liquid nitrogen. The micro-powder was compressed uniaxially to form rock ice. Ice sphere characteristics give some insights into very low temperature phase transformations in ice. A cooling system based on the thermoelectric properties of the peltier chip was developed to apply the heat treatments (min T = -22C). The grain size data was collected qualitatively, using cryo-EBSD. Both sensible (1/n > 1) and nonsensical (1/n < 1) exponents were found. The sensible data gave inverse exponents between 2 and 40, which increased with porosity. Growth rates in my synthetic porous ices are less than those for fully dense, pure synthetic ice and greater than the rates observed in terrestrial ice and firn. The nonsensical exponentials were found in experiments where the two samples grew according to different kinetics. This violated the underlying assumption of the mathematical reasoning of the method. The exponents found compare favourably with those found with the same data using the traditional method to determine n. The exponents vary significantly with the different averaging tool used to determine grain size. The largest source of error in the proposed methodology stems from the non-similarity of the paired samples. There is a significant portion of error from measurement errors in D. This makes for imprecise exponents. The imprecision in n values found with this method is also present in n constrained using the traditional method. Care is required when quoting growth rates in terms of n

New grain growth experiments in water ice

Grain growth exponents are constrained independently of the thermally activated kinetics of growth. Two homogeneous samples with dissimilar grain sizes were subjected to a single heating event. The exponential n is proportional to the ratio of their change in grain size (D) (see thesis for equation). Heat treatment experiments were performed in superfine (< 25um) ice samples of variable porosity. A new type of superfine ice was synthesised. Spherical ice particles (2-30um) were generated by using an inkjet piezoelectric printer to spray micro-droplets of deionised water into liquid nitrogen. The micro-powder was compressed uniaxially to form rock ice. Ice sphere characteristics give some insights into very low temperature phase transformations in ice. A cooling system based on the thermoelectric properties of the peltier chip was developed to apply the heat treatments (min T = -22C). The grain size data was collected qualitatively, using cryo-EBSD. Both sensible (1/n > 1) and nonsensical (1/n < 1) exponents were found. The sensible data gave inverse exponents between 2 and 40, which increased with porosity. Growth rates in my synthetic porous ices are less than those for fully dense, pure synthetic ice and greater than the rates observed in terrestrial ice and firn. The nonsensical exponentials were found in experiments where the two samples grew according to different kinetics. This violated the underlying assumption of the mathematical reasoning of the method. The exponents found compare favourably with those found with the same data using the traditional method to determine n. The exponents vary significantly with the different averaging tool used to determine grain size. The largest source of error in the proposed methodology stems from the non-similarity of the paired samples. There is a significant portion of error from measurement errors in D. This makes for imprecise exponents. The imprecision in n values found with this method is also present in n constrained using the traditional method. Care is required when quoting growth rates in terms of n

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled

Extreme hydrothermal conditions at an active plate-bounding fault

International audienceTemperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes1. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre2, 3. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades4, 5. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled