Hematite Helium Dating of Bedrock Fractures and Faults (HeHe BFF)

Despite their importance for hydrologic and geophysical properties of the shallow crust, the age and origins of bedrock-hosted fractures, faults, and veins are often difficult to determine. This is partly because existing geochronologic methods are not very good at dating the limited range of secondary phases that form in them. Fe(III)-oxides, especially hematite, are extremely common in such features in shallow crustal rocks, where their formation is associated with fluid flow and deformation episodes occurring long after host rock formation. The (U-Th)/He system poses potential for dating these secondary Fe-oxides because they contain variable but often high (often tens to hundreds of ppm) concentrations of U and/or Th and little to no initial He. Hematite (U-Th)/He (HeHe) dates from bedrock faults and fractures (BFF) (and veins) in several geologic settings illustrate the potential of this approach for understanding a range of phenomena related to fluid flow and deformation. For example, thin (~1-5 mm) hematite veins in Proterozoic crystalline basement from four locations in central Arizona have ages of HeHe dates of 425-525 Ma, 570-630 Ma, 750-930 Ma, and 800-900 Ma. These could be interpreted as the timing of vein formation during the penultimate near-surface exhumation episode (the last one being that which exhumed them in the late Cenozoic) that exposed these rocks to migrating oxidized groundwater. Not all hematite veins in crystalline basement are that old, however, and in some cases their ages may be easier to associate with regional tectonic events. A thin hematite vein in the Boulder Creek batholith just below the basal Fountain Formation yields HeHe ages of 171 ± 4.6 Ma. This corresponds to a major unconformity in the overlying sedimentary sequence, and a sharp increase in basin subsidence and magmatic input in the Utah-Idaho trough/Arapien basin several hundred kilometers to the west. Similarly, in the Galiuro Mountains of southern Arizona, 1.1-Ga basement contains two types of veins: planar, pure 800-900 Ma hematite veins, and ~90-100-Ma, en echelon tensional quartz-hematite veins. In both of settings, these Mesozoic ages are significantly earlier than the expected arrival age of Cordilleran contraction, but they may represent fluid infiltration associated with minor uplift and extension on a migrating forebulge (a la Oliver, 1986). Elsewhere, HeHe dates from hematite breccias and in faults in the Santa Catalina core complex yield ages of ~9-13, consistent with the latest phase of high-angle normal faulting in the range. Supergene-alteration hematite in another nearby extensional block is 15 Ma, matching the timing of footwall exhumation. And hematite formed along slickensides in minor faults in the Fish Canyon Tuff are 4-6 Ma, which may indicate recent minor deformation. One setting where we find anomalously young hematite ages is in bedrock fractures is in quartzite of the Stansbury range of Utah. Here, ages are correlated with elevation, ranging from 2-3 Ma near the summit to 700 ka near the base of the range. Lack of ferrous primary minerals in this bedrock, or shallow oxide formation combined with glacial erosion may explain the relatively young ages here.