Alexis K. Ault

Hot faults: Iridescent slip surfaces with metallic luster document high- temperature ancient seismicity in the Wasatch fault zone, Utah, USA

and localized iridescence in the footwall dam- age zone of the Wasatch fault (Utah, USA) and document elevated temperatures on these faults. We propose that the formation of these iridescent slip surfaces requires frictional heat generated at geometric asperities at seismic slip rates. WASATCH FAULT ZONE ABSTRACT We document new geological indicators of ancient seismicity in the form of highly reflec- tive, iridescent, hematite-coated fault surfaces. Small faults that cut the Paleoproterozoic Farmington Canyon Complex in the footwall damage zone of the Brigham City segment of the Wasatch fault (Utah, USA) are smooth to striated surfaces, tens of square centimeters to 30 m 2 in area. The dull-rusty to high-metallic luster and moderate- to high-gloss surfaces exhibit multicolored elliptical iridescent patches ~0.5-3 cm across. Preexisting hematite crystals were deformed during slip on 1-200-mm-thick slip surfaces. Textural observations, X-ray diffraction, X-ray photoelectron spectroscopy, electron backscattered diffraction analysis, surface metrology, and similarity to experimentally formed iridescent spots in rocks and metals indicate that iridescence is associated with high-temperature (>300 °C) reduction of iron (Fe 3+ to Fe 2+ ) and associated conversion of hematite to magnetite. We pro- pose that the iridescent slip surfaces in the Wasatch fault damage zone are the result of seismic slip and flash heating at asperities along the small faults. The thousands of these sur - faces represent coseismic or aftershock deformation down to magnitude -3 in the exhumed footwall damage zone of the Wasatch fault.

Record of paleofluid circulation in faults revealed by hematite (U-TH)/He and apatite fission-track dating: an example from Gower Peninsula Fault Fissures, Wales

Fault rock low-temperature thermochronometry can inform the timing, temperature, and significance of hydrothermal fluid circulation in fault systems. We demonstrate this with combined hematite (U-Th)/He (He) dating, and sandstone apatite fission-track (AFT) and apatite and zircon (U-Th)/He (He) thermochronometry from fault-related fissures on the Gower Peninsula, Wales. Hematite He dates from 141 ± 5.1 Ma to 120 ± 5.0 Ma overlap with a 131 ± 20 Ma sandstone infill AFT date. Individual zircon He dates are 402–260 Ma, reflecting source material erosion, and imply a maximum Late Permian infill depositional age. Burial history reconstruction reveals modern exposures were not buried sufficiently in the Triassic–Early Cretaceous to have caused reheating to temperatures necessary to reset the AFT or hematite He systems, and thus these dates cannot reflect cooling due to erosion alone. Hot fluids circulating through fissures in the Early Cretaceous reset the AFT system. Hematite was either also reset by fluids or precipitated from these fluids. Similar hematite He dates from fault-related mineralization in south Glamorgan (Wales) and Cumbria (England) imply concomitant regional hot groundwater flow along faults. In this example, hydrothermal fluid circulation, coeval with North Atlantic rifting, occurred in higher-permeability fissures and fault veins long after they initially formed, directly influencing local and regional geothermal gradients.

Linking hematite (U-Th)/He dating with the microtextural record of seismicity in the Wasatch fault damage zone, Utah, USA

Techniques directly dating fault slip are few, limiting the ability to interpret the rock record of seismicity. Hematite is commonly found in fault zones, amenable to (U-Th)/He dating, and slip surface hematite may be reset by shear heating events and/or recrystallization. Glossy hematite-coated fault surfaces in the Wasatch fault footwall damage zone, Utah (USA), exhibit evidence of hematite cataclasis and preserve Pliocene hematite (U-Th)/He dates. Apatite (U-Th)/He and fission track data from the host gneiss indicate footwall unroofing through ∼2 km by ca. 4.5 Ma. Internally reproducible but disparate hematite (U-Th)/He dates 4.5 Ma and younger from isolated locations on a single fault surface do not reflect ambient cooling. We hypothesize that these dates, and associated iridescence and annealed crystal texture, document rapid cooling from friction-generated heat during small seismic slip events between ca. 4.5 and 2.5 Ma. Thus, hematite (U-Th)/He dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 10 5 –10 6 yr time scales.

Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

The study of deeply exhumed ancient collisional belts offers important constraints on geologic processes and properties complementary to inaccessible portions of the crustal column in active orogens. The ca. 1.8−1.7 Ga Big Sky orogeny in southwest Montana is a major convergent belt associated with the Proterozoic amalgamation of Laurentia. New structural, petrologic, and geochronologic data from the Northern Madison Range, crossing the NE-SW trend of the belt, record key information about the internal dynamics of the orogen. At least two phases of Big Sky−related deformation are preserved, both nearly coeval with peak metamorphic conditions of ∼0.9−0.8 GPa and >700 °C. Metamorphic zircon grains from a deformed mafic dike yield a weighted mean ion probe U-Pb date of 1737 ± 28 Ma (2σ). Monazite grains from a metapelite yield electron microprobe U-Th total-Pb dates of ca. 1750−1705 Ma, spanning prograde, peak, and retrograde intervals. Exposed Proterozoic paleodepths range from deeper levels (∼45−40 km; 1.2 GPa) in the northwestern end of the range to shallower levels (∼30−25 km) in the central-southeast area. The age of high-grade tectonism appears to become younger southeastward away from the core of the orogen, from ca. 1810−1780 Ma in the Highland Mountains, to ca. 1780−1750 Ma in the Ruby Range, Tobacco Root Mountains, and northwesternmost Northern Madison Range, and 1750−1720 Ma in the central Northern Madison Range. These spatial and temporal patterns of lateral growth and propagation of the orogen are similar to those observed in other collisional orogenic systems, and they may reflect multiple collision phases, protracted collision, and/or postcollisional collapse.