No abstract

No abstract

No abstract

The fi rst phase of the Deep Fault Drilling Project (DFDP-1) yielded a continuous lithological transect through fault rock surrounding the Alpine fault (South Island, New Zealand). This allowed micrometer-to decimeter-scale variations in fault rock lithology and structure to be delineated on either side of two principal slip zones intersected by DFDP-1A and DFDP-1B. Here, we provide a comprehensive analysis of fault rock lithologies within 70 m of the Alpine fault based on analysis of hand specimens and detailed petrographic and petrologic analysis. The sequence of fault rock lithologies is consistent with that inferred previously from outcrop observations, but the continuous section afforded by DFDP-1 permits new insight into the spatial and genetic relationships between different lithologies and structures. We identify principal slip zone gouge, and cataclasite-series rocks, formed by multiple increments of shear deformation at up to coseismic slip rates. A 20-30-m-thick package of these rocks (including the principal slip zone) forms the fault core, which has accommodated most of the brittle shear displacement. This deformation has overprinted ultramylonites deformed mostly by grain-size-insensitive dislocation creep. Outside the fault core, ultramylonites contain low-displacement brittle fractures that are part of the fault damage zone. Fault rocks presently found in the hanging wall of the Alpine fault are inferred to have been derived from protoliths on both sides of the present-day principal slip zone, specifi cally the hanging-wall Alpine Schist and footwall Greenland Group. This implies that, at seismogenic depths, the Alpine fault is either a single zone of focused brittle shear that moves laterally over time, or it consists of multiple strands. Ultramylonites, cataclasites, and fault gouge represent distinct zones into which deformation has localized, but within the brittle regime, particularly, it is not clear whether this localization accompanies reductions in pressure and temperature during exhumation or whether it occurs throughout the seismogenic regime. These two contrasting possibilities should be a focus of future studies of fault zone architecture.

Interseismic recovery of fault strength (healing) following earthquake failure is a fundamental requirement of the seismic cycle and likely plays a key role in determining the stability and slip behavior of tectonic faults. We report on laboratory measurements of time‐ and slip‐dependent frictional strengthening for natural and synthetic gouges to evaluate the role of mineralogy in frictional strengthening. We performed slide‐hold‐slide (SHS) shearing experiments on nine natural fault gouges and eight synthetic gouges at conditions of 20 MPa normal stress, 100% relative humidity (RH), large shear strain (~15), and room temperature. Phyllosilicate‐rich rocks show the lowest rates of frictional strengthening. Samples rich in quartz and feldspar exhibit intermediate rates of frictional strengthening, and calcite‐rich gouges show the largest values. Our results show that (1) the rates of frictional strengthening and creep relaxation scale with frictional strength, (2) phyllosilicate‐rich fault gouges have low strength and healing characteristics that promote stable, aseismic creep, (3) most natural fault gouges exhibit intermediate rates of frictional strengthening, consistent with a broad range of fault slip behaviors, and (4) calcite‐rich fault rocks show the highest rates of frictional strengthening, low values of dilation upon reshear, and high frictional strengths, all of which would promote seismogenic behavior.

The strength of tectonic faults and the processes that control earthquake rupture remain central questions in fault mechanics and earthquake science. We report on the frictional strength and constitutive properties of intact samples across the main creeping strand of the San Andreas fault (SAF; California, United States) recovered by deep drilling. We fi nd that the fault is extremely weak (friction coeffi cient, µ = ~ 0.10), and exhibits both velocity strengthening frictional behavior and anomalously low rates of frictional healing, consistent with aseismic creep. In contrast, wall rock to the northeast shows velocity weakening frictional behavior and positive healing rates, consistent with observed repeating earthquakes on nearby fault strands. We also document a sharp increase in strength to values of µ > ~0.40 over <1 m distance at the boundary between the fault and adjacent wall rock. The friction values for the SAF are suffi ciently low to explain its apparent weakness as inferred from heat fl ow and stress orientation data. Our results may also indicate that the shear strength of the SAF should remain approximately constant at ~10 MPa in the upper 5-8 km, rather than increasing linearly with depth, as is commonly assumed. Taken together, our data explain why the main strand of the SAF in central California is weak, extremely localized, and exhibits aseismic creep, while nearby fault strands host repeating earthquakes.

[1] New laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick-slip motion of debris-laden ice over rock, with implications for glacier behavior. Friction between a rock substrate and clasts in ice generates heat, which melts nearby ice to produce lubricating water. An increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production. Unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase. Low initial lubrication can result from cold or drained conditions, whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads. Strengthening of the interface (healing) during "stick" intervals between slip events occurs primarily through meltwater refreezing. When healing and unstable sliding are taken together, the experiments reported here suggest that stick-slip behavior is common from motion of debris-laden glacier ice over bedrock.

[1] We study the mechanisms of frictional strength recovery for tectonic faults with particular focus on fault gouge that contains phyllosilicate minerals. We report laboratory and microstructural work from fault rocks associated with a regional, low-angle normal fault in Central Italy. Experiments were conducted in a biaxial deformation apparatus at room temperature and humidity, nominally dry, under constant normal stresses of 20 and 50 MPa, and at a sliding velocity of 10 mm/s. Our results for nominally dry conditions show good agreement with previous work conducted under controlled pore fluid pressure. The phyllosilicate contents of our samples, which include clay, talc and chlorite range from 0 to 52 weight %. We study both intact rock samples, sheared in their in situ geometry, and powders made from the same rocks to address the role of fabric in fault healing. We measured frictional healing, Dm, using slide-hold-slide tests with hold periods ranging from 3 to 3000 s. Phyllosilicate-free materials show friction values of m ≈ 0.6 and healing rates that are larger in powdered samples, b ≈ 0.006 (Dm per decade in time, s) compared to intact wafers of fault rock, b ≈ 0.004. For phyllosilicate-bearing materials, healing rates are low, b < 0.002, and independent of fabric, phyllosilicate content and normal stress. We observe that frictional strength decreases systematically with increasing phyllosilicate content. Intact, phyllosilicate-bearing fault rock is consistently weaker than its powdered equivalent (0.2 < m < 0.3 versus 0.4 < m < 0.5, respectively). We compare our data to results from experiments conducted on a wide range of materials and conditions. Deformation microstructures show localized slipping along sub-parallel shear planes. We suggest that low values of frictional strength and near zero healing rates will combine to exacerbate the weakness of phyllosilicate-bearing faults and promote stable, aseismic creep.Citation: Tesei, T., C. Collettini, B. M. Carpenter, C. Viti, and C. Marone (2012), Frictional strength and healing behavior of phyllosilicate-rich faults,