Determination of volume loss and element mobility patterns associated with the development of the Copper Basin fault, Picacho State Recreation Area, SE California, USA

• We provide a detailed discussion of the architectural elements of the Copper Basin fault zone.
• Significant losses of elemental mass, bulk mass, and volume within the fault core.
• Our data implies higher fluid-to-rock ratios in the fault core than the adjacent damage zone.
• During rupture, the permeability within the fault core must have increased significantly.
• The Copper Basin fault may have formed through the reactivation of an older fault.

The fault zone architectural elements of the Copper Basin fault include a fault core, inner damage zone, and outer damage zone. Utilizing Yb as a reference frame element, significant elemental mass changes were documented across all three architectural zones, with the greatest change in the fault core. Porosity and volumetric strain both increase toward the principal slip surface reaching a maximum in the inner damage zone and then dropping significantly in the fault core. In the damage zone and unaffected wall rocks, smectite is the most prominent clay species, whereas illite dominates the fault core suggesting that temperatures may have reached ∼100–150 °C or more during the production of the fault core. A number of studies have interpreted similar mass changes and clay mineral reactions to indicate high water/rock ratios. Such interpretations imply that during rupture, permeability increased within the fault core allowing it to act as a pathway for hot, chemically reactive fluids. Then during and after rupture, CO2 and SiO2 rich fluids moved into the inner and outer damage zones where calcite and quartz precipitation healed the fractured and permeable rock.