Document Type


Journal/Book Title/Conference

American Geophysical Union

Publication Date



We performed systematic structural and geochemical analyses on a suite of cored rocks from the vertical Cajon Pass, California drill hole to characterize the deformation and alteration of fault-related rocks. The drill hole lies 4 km northeast of the San Andreas Fault (SAF), and observations of deformed crystalline rock in core and outcrop provide a sample of a 5-km vertical column adjacent to the steeply dipping Cleghorn fault and span the brittle to semi-brittle deformational regime at hydrothermal conditions. The rocks in the upper 500 m of the borehole are composed of sandstones and granitoid augen gneiss, with narrow fault and fracture zones coated with thin seams of laumontite. Below 500 m depth in the core, tonalite gneiss and migmatite contain well-developed discrete brittle faults and fracture zones. Thirty-seven faults are recognized in the core and borehole data; eleven are newly identified here, eight were previously identified in the core, and the remainder are interpreted from borehole image log data. The size of the fault zones intersected by the core controls the extent and nature of deformation. Distribution of faults in the core increase with depth, and fracture densities are greater around fault zones. In the upper 2600 m of the hole, the faults and fractures are typically narrow with thin coatings of alteration products. Prominent fault zones at 2100 - 2300 and 2500 - 2600 m measured depth dip moderately to steeply, and within this fault distributed shearing and alteration textures are common. Microstructures in these fault zones primarily include shear fractures containing a matrix of laumontite with angular to sub-angular clasts within the matrix and record multiple cycles of deformation and alteration. Laumontite mineralization indicates moderate- to high-temperature fluids interacting with the rocks throughout most of the column. The most significant fault observed in the core is an indurated, steep-dipping zone at 3,402 m depth that exhibits evidence of a mixture of brittle and semi-brittle deformation and abundant mineralization and alteration of potassium feldspar and epidote. This fault correlates well with the left-lateral steeply dipping Cleghorn fault, and reflects the interaction between hydrothermal metasomatic alteration and brittle fracture, cataclastic flow, to incipient plastic deformation processes at depth. The interpretation that the fault zone at the bottom of the hole is the Cleghorn fault agrees with stress orientation measurements made there by M D. Zoback and coworkers and indicates that the faults in the drill hole reflect active deformation and alteration associated with northeast-oriented maximum horizontal stress that may drive the left-lateral oblique motion on the Cleghorn fault. The data also show that damage zones associated with faults are present here, and may consist of mixed mode deformation, indicating a long-lived presence of the deformed and altered zones of reduced elastic moduli associated with faults. Simple modeling of the thermodynamics of syntectonic reactions in the fault zones indicate that earthquakes can be the source of heat to drive the reactions, and thus earthquake energy may be consumed in the fault core and damage zone by focused alteration.