A series of hydrothermal frictional strength experiments was conducted on synthetic gouges prepared from five chlorite-rich samples of varying composition. We present chlorite mineral compositions obtained by electron microprobe techniques, and X-ray diffraction and bulk chemical analyses of the gouges that were prepared by hand-grinding each sample to less than 90-µm grain diameter. Major and minor element gouge analyses were conducted in the Geology, Geophysics, and Geochemistry Science Center laboratory of the U. S. Geological Survey in Lakewood, CO. All of the other data were collected in the earthquake Science Center laboratories, which were located at that time in Menlo Park, CA. Chlorite samples tested: Fe-chlorite. Chlorite-garnet schist of unknown origin. The millimeter-scale euhedral garnets were removed from the sample prior to grinding. Ore14-1. Chlorite blackwall, the inner zone of a metasomatic alteration rind surrounding a high-grade mafic blueschist block in Coos Counry, southwest Oregon. Location: 42.9883°N, 124.3811°W. LP14-3a. Large, chlorite-rich clast exposed in an exhumed shear on the west side of Lake Pillsbury, Mendocino County, California. Location: 39.4072°N, 122.9575°W. CCa-2. Ripidolite chlorite, obtained from the Clay Minerals Society Special Clay Minerals Collection. Flagstaff Hill, El Dorado County, California. Location: 38.7648°N, 121.0839°W. Russian chlorite. Clinochlore cleavage flakes from Achmativ Mine, a famous skarn deposit located near Zlatonst in the Southern Urals, Russia. Location: 55.3042°N, 59.6561°E.

We present whole-rock geochemical analyses of 12 core samples obtained from three serpentinite mud volcanoes (Yinazao, Asut Tesoru, and Fantangisna) located on the forearc of the Mariana subduction system, where the Pacific Plate descends beneath the Philippine Sea Plate. The core was collected during International Ocean Discovery Program Expedition 366 of 2016-2017. The materials comprising the mud volcanoes have risen diapirically along normal faults in the forearc that may extend down to the subducting slab. Ten of the samples are thoroughly serpentinized ultramafic rocks. The other two come from cored intervals into Fantangisna mud volcano that contain materials derived from the subducting Pacific Plate, and their compositions are consistent with the inclusion of a substantial crustal component. The major, trace, and rare-earth element data reported for each analyzed core sample were obtained at the GeoAnalytical Lab of Washington State University, Pullman, Washington.

Using laboratory slide-hold-slide experiments, at temperatures from 22 to 200 degrees C, to examine effects of fracture reactivation and quasi-static loading on the evolution of fluid transport properties of simulated fractures in Westerly granite. At all temperatures, the in-plane hydraulic transmissivity consistently decays during hold periods resulting in an overall reduction in transmissivity. During the first three to fifteen hours of an experiment, transmissivity decreases rapidly due to the generation of wear products, development of a sliding surface, and compaction of the resulting gouge. Once the sliding surface has developed, the long-term transmissivity decay rate at 22 and 100 degrees C is significantly lower than the transmissivity decay rate during the initial 3-15 hours of the experiment. However, at 200 degrees C, the decay of hydraulic transmissivity remains high throughout the experiment. The long-term decay of hydraulic transmissivity can be fitted with a power law model with more rapid reduction of hydraulic transmissivity at higher temperature. Periods of sliding on the fracture surface result in transient increases in the transmissivity, due to shear dilation, as is expected for Coulomb materials. These transients are superimposed on the long-term decay. When sliding ceases and a new hold period commences, there is a rapid reduction in transmissivity and return to the long-term rate of transmissivity decay. The rate of decay of the transmissivity transients is inversely correlated with temperature, in contrast to the long-term decay and the expected behavior for processes like subcritical crack growth and indentation creep. The higher decay rates that are observed during the initial 3-15 hours of the tests and following sliding, are associated with times that the porosity of the gouge is expected to be high. The difference in decay rates suggests that when the gouge is driven far from equilibrium by active shearing, densification may be dominated by a different mechanism from long-term compaction.

Laboratory slide-hold-slide tests were conducted in a conventional triaxial deformation configuration on 1-inch diameter cylindrical cores of Westerly granite bisected by a sawcut oriented at 30 degrees from vertical. Tests were conducted at a constant confining pressure of 30 MPa with a 10 MPa pore fluid pressure. The pore fluid was deionized water. Experiments were conducted at temperatures of 22, 100, 200, and 250 degC.

Laboratory slide-hold-slide tests were conducted in a conventional triaxial deformation configuration on 1-inch diameter cylindrical cores of Westerly granite bisected by a sawcut oriented at 30 degrees from vertical. Tests were conducted at a constant confining pressure of 30 MPa with a 10 MPa pore fluid pressure. The pore fluid was deionized water. Experiments were conducted at temperatures of 22, 100, 200, and 250 C. This data was collected to examine fault strength recovery in hydrothermal conditions.

We present whole-rock geochemical analyses of 27 Phase 3 core samples obtained from Hole G of the San Andreas Fault Observatory at Depth, which crosses the creeping section of the San Andreas Fault ~14 km NW of Parkfield, California. These new data will essentially double the published whole-rock chemical dataset for the SAFOD Hole G core. Two, meters-wide zones of foliated gouge where fault creep is concentrated were successfully cored in Hole G. The gouge zones are bounded on either side by variably deformed structural units of sedimentary rock. Sampling for this dataset was focused on the gouge-wall rock boundaries and meters-long sections of the wall rocks that previously lacked chemical data. Major, trace, and rare-earth element data were obtained for each analyzed core sample at the GeoAnalytical Lab of Washington State University, Pullman, Washington.

This report presents the mineral chemistry dataset that was used in a published study of serpentinite-rich gouge from an actively creeping trace of the Bartlett Springs Fault in northern California. The fault gouge consists of porphyroclasts of antigorite serpentinite, talc, chlorite, and tremolite-actinolite in a sheared matrix of the same materials. The compositions of spinels in the serpentinite clasts (Table S1) indicate a forearc-peridotite origin for the serpentinite, consistent with a source in the Coast Range ophiolite. The other major minerals in both the porphyroclasts and matrix were analyzed (Table S2) for comparison with serpentinite-bearing gouge from the creeping section of the San Andreas Fault. Table S3 presents serpentine mineral compositions from outcrops of the Coast Range Ophiolite located away from the fault.

This data release comprises three separate datasets and their accompanying metadata, in zip files. The data were acquired as part of a laboratory study of the response of ultramafic materials to shear at hydrothermal conditions. The principal dataset consists of the strength-displacement data from 28 friction experiments acquired on gouges prepared from peridotite rock samples and from separates of its principal mineral constituents olivine and orthopyroxene. The other two datasets present mineralogical data for the run products obtained by X-ray diffraction (XRD) and energy dispersive system (EDS) techniques.

Core samples from the International Ocean Discovery Program (IODP) Expedition 366 were tested in the laboratory to determine permeability, porosity, density, and frictional strength and their relation to mineralogy as part of an effort to understand hydro-mechanical processes at convergent plate margins. Seven samples were tested from a depth range of 19.6 to 197.9 m below the sea floor. The samples were derived from three serpentinite mud volcanoes in the Mariana forearc region, formed where slab-derived fluids and materials ascend along faults. The physical characteristics mirror compositional differences between predominantly serpentine-rich and saponite-rich samples. Permeability values ranged from 10-17 to 10-19 m2, low enough to facilitate the formation of high fluid pressures, which have been observed in the Mariana and other subduction megathrust environments. Porosities ranged from 0.37 to 0.51 and densities from 1.66 to 2.01 gm/cc. Serpentine-rich samples have coefficients of friction of 0.2 to 0.4, consistent with crustal serpentinite from a variety of fault zones, whereas saponite-rich samples have friction values below 0.2, consistent with saponite fault gouge from the San Andreas Fault Zone at Depth (SAFOD) drillhole in California.

New Zealand's Alpine Fault (AF) ruptures quasi-periodically in large-magnitude earthquakes. Paleoseismological evidence suggests that about half of all recognized AF earthquakes terminated at the boundary between the Central and South Westland sections of the fault. There, fault geometry and the polarity of uplift change. The South Westland AF exhibits oblique-normal fault motion on a structure oriented 055/82SE that, for at least 35 km along strike, contains saponite-rich principal slip zone gouges. New hydrothermal friction experiments reveal that the saponite fault gouge is frictionally weak, exhibiting friction coefficients between ?=0.12 and ?=0.16 for a range of temperatures (T=25-210 C) and effective normal stresses (?n'=31.2-93.6 MPa). The saponite gouge is rate-strengthening in all velocity steps performed at velocities between 0.01 and 3.0 ?m/s, behavior conducive to aseismic creep. A three-dimensional stress analysis shows that the South Westland AF is favorably oriented with respect to the regional stress field for slip within the frictionally weak saponite fault gouge. Geometrically, the fault is severely misoriented for slip in any fault-forming materials with friction coefficients exceeding ?~0.5. The combination of weak gouges prone to aseismic creep, strong asperities, and low resolved shear stress may impede earthquake rupture propagation along the South Westland Alpine Fault.