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1. Seismicity recorded in hematite fault mirrors in the Rio Grande rift

   https://doi.org/10.1130/GES02426.1  

   Odlum, M.L.; Ault, A.K.; Channer, M.A.; Calzolari, G. (November 2021, Geosphere)

   Abstract Exhumed fault rocks provide a textural and chemical record of how fault zone composition and architecture control coseismic temperature rise and earthquake mechanics. We integrated field, microstructural, and hematite (U-Th)/He (He) thermochronometry analyses of exhumed minor (square-centimeter-scale surface area) hematite fault mirrors that crosscut the ca. 1400 Ma Sandia granite in two localities along the eastern flank of the central Rio Grande rift, New Mexico. We used these data to characterize fault slip textures; evaluate relationships among fault zone composition, thickness, and inferred magnitude of friction-generated heat; and document the timing of fault slip. Hematite fault mirrors are collocated with and crosscut specular hematite veins and hematite-cemented cataclasite. Observed fault mirror microstructures reflect fault reactivation and strain localization within the comparatively weaker hematite relative to the granite. The fault mirror volume of some slip surfaces exhibits polygonal, sintered hematite nanoparticles likely created during coseismic temperature rise. Individual fault mirror hematite He dates range from ca. 97 to 5 Ma, and ~80% of dates from fault mirror volume aliquots with high-temperature crystal morphologies are ca. 25–10 Ma. These aliquots have grain-size–dependent closure temperatures of ~75–108 °C. A new mean apatite He date of 13.6 ± 2.6 Ma from the Sandia granite is consistent with prior low-temperature thermochronometry data and reflects rapid, Miocene rift flank exhumation. Comparisons of thermal history models and hematite He data patterns, together with field and microstructural observations, indicate that seismicity along the fault mirrors at ~2–4 km depth was coeval with rift flank exhumation. The prevalence and distribution of high-temperature hematite grain morphologies on different slip surfaces correspond with thinner deforming zones and higher proportions of quartz and feldspar derived from the granite that impacted the bulk strength of the deforming zone. Thus, these exhumed fault mirrors illustrate how evolving fault material properties reflect but also govern coseismic temperature rise and associated dynamic weakening mechanisms on minor faults at the upper end of the seismogenic zone. 

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2. Hematite Frictional Behavior and He Loss From Comminution During Deformation Experiments at Slow Slip Rates

   https://doi.org/10.1029/2023JB027514  

   DiMonte, A A; Ault, A K; Hirth, G; Meyers, C D (March 2024, Journal of Geophysical Research: Solid Earth)

   Deformation experiments on hematite characterize its slip‐rate dependent frictional properties and deformation mechanisms. These data inform interpretations of slip behavior from exhumed hematite‐coated faults and present‐day deformation at depth. We used a rotary‐shear apparatus to conduct single‐velocity and velocity‐step experiments on polycrystalline specular hematite rock (∼17 μm average plate thickness) at slip rates of 0.85 μm/s to 320 mm/s, displacements of primarily 1–3 cm and up to 45 cm, and normal stresses of 5 and 8.5 MPa. The average coefficient of friction is 0.70; velocity‐step experiments indicate velocity‐strengthening to velocity‐neutral behavior at rates <1 mm/s. Scanning electron microscopy showed experimentally generated faults develop in a semi‐continuous, thin layer of red hematite gouge. Angular gouge particles have an average diameter of ∼0.7 μm, and grain size reduction during slip yields a factor of 10–100 increase in surface area. Hematite is amenable to (U‐Th)/He thermochronometry, which can quantify fault‐related thermal and mechanical processes. Comparison of hematite (U‐Th)/He dates from the undeformed material and experimentally produced gouge indicates He loss occurs during comminution at slow deformation rates without an associated temperature rise required for diffusive loss. Our results imply that, in natural fault rocks, deformation localizes within coarse‐grained hematite by stable sliding, and that hematite (U‐Th)/He dates acquired from ultracataclasite or highly comminuted gouge reflect minor He loss unrelated to thermal processes. Consequently, the magnitude of temperature rise and associated thermal resetting in hematite‐bearing fault rocks based on (U‐Th)/He thermochronometry may be overestimated if only diffusive loss of He is considered. 

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3. Signatures of Fluid-Rock Interactions in Shallow Parts of the San Andreas and San Gabriel Faults, Southern California

   https://doi.org/10.55575/tektonika2024.2.2.47  

   Evans, James; Crouch, Kaitlyn; Studnicky, Caroline; Bone, Sharon; Edwards, Nicholas; Webb, Samuel (January 2024, Tektonika)

   We examine deformed crystalline bedrock in the upper parts of the active San Andreas and ancient San Gabriel Faults, southern California, to 1) determine the nature and origin of micro-scale composition and geochemistry of fault-related rocks, 2) constrain the extent of fluid-rock interactions, and 3) determine the interactions between alteration, mineralization, and deformation. We used drill cores from a 470 m long inclined borehole through the steep-dipping San Gabriel Fault and from seven inclined northeast-plunging boreholes across the San Andreas Fault zone to 150 m deep to show that narrow fault cores 10 cm to 5 m wide lie within 100s m wide damage zones. Petrographic, mineralogic, whole-rock geochemical analyses and synchrotron-based X-ray fluorescence mapping of drill core and thin sections of rocks from the damage zone and narrow principal slip surfaces reveal evidence for the development of early fracture networks, with iron and other transition element mineralization and alteration along the fractures. Alteration includes clay $$\pm$$ chlorite development, carbonate, and zeolite mineralization in matrix and fractures and the mobility of trace and transition elements. Carbonate-zeolite mineralization filled fractures and are associated with element mobility through the crystalline rocks. Textural evidence for repeated shearing, alteration, vein formation, brittle deformation, fault slip, pressure solution, and faulted rock re-lithification indicates significant hydrothermal alteration occurred during shallow-level deformation in the fault zones. The rock assemblages show that hydrothermal conditions in active faults develop at very shallow levels where seismic energy, heat, and fluids are focused. 

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4. Thermochronometric and textural evidence for seismicity via asperity flash heating on exhumed hematite fault mirrors, Wasatch fault zone, UT, USA

   McDermott, R. G. (January 2017, Earth and planetary science letters)

   Exhumed faults record the temperatures produced by earthquakes. We show that transient elevated fault surface temperatures preserved in the rock record are quantifiable through microtextural analysis, fault-rock thermochronometry, and thermomechanical modeling. We apply this approach to a network of mirrored, minor, hematite-coated fault surfaces in the exhumed, seismogenic Wasatch fault zone, UT, USA. Polygonal and lobate hematite crystal morphologies, coupled with hematite (U–Th)/He data patterns from these surfaces and host rock apatite (U–Th)/He data, are best explained by friction-generated heat at slip interface geometric asperities. These observations inform thermomechanical simulations of flash heating at frictional contacts and resulting fractional He loss over generated fault surface time–temperature histories. Temperatures of >∼700–1200 °C, depending on asperity size, are sufficient to induce 85–100% He loss from hematite within 200 μm of the fault surface. Spatially-isolated, high-temperature microtextures imply spatially-variable heat generation and decay. Our results reveal that flash heating of asperities and associated frictional weakening likely promote small earthquakes (Mw≈−3 to 3) on Wasatch hematite fault mirrors. We suggest that similar thermal processes and resultant dynamic weakening may facilitate larger earthquakes. 

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5. Evaluating the Compatibility of Hematite (U‐Th)/He Data and Hematite‐Carried Secondary Magnetizations: An Example From the Colorado Front Range

   https://doi.org/10.1029/2023GC010993  

   Jensen, Jordan L; Ault, Alexis K; Geissman, John W (September 2023, Geochemistry, Geophysics, Geosystems)

   Ancient magnetization(s), often recorded by hematite (Fe₂O₃), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations. 

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