OBSERVATIONS FROM THE CAJON PASS CRYSTALLINE CORE, CALIFORNIA

We revisit the drilled crystalline core from the Cajon Pass, California drill hole, along the San Andreas (SAF), and Cleghorn faults, to perform a systematic structural analysis of deformation and alteration associated with strike slip faulting at the site. Previous lithologic descriptions of the core did not incorporate descriptions or interpretations of deformation processes at depth. The core and outcrop observations provide a sampling of a 4.5 km vertical column adjacent to the SAF. The shallowest level of deformation is represented by damage associated with the Cleghorn fault, which is composed of a > 100 m wide zone of brittle fractures and limited mineralization. Shallow rocks in the borehole are predominantly sandstones and augen granites, with few fault and fracture zones. Deeper in the core, gneisses, granite diorites, and granite gneisses dominate, and intense faults and fracture zones are present with epidote and potassium feldspar alterations. Damaged zones from deeper in the core have more intense potassium feldspar alteration. Fractures in the core include predominantly epidote, potassium feldspar alterations, as well as zeolites associated with fractures located throughout the core. Measurements of fracture length, thickness, and angle were taken throughout the all of the crystalline core. Fracture patterns, intensity, and fill were noted to determine where fault zones correlate with geophysical data. Several fault zones were preserved by the core, including a fully intact fault zone at 11,162 ft depth with potassium feldspar and epidote alteration. The fault zone appears to dip steeply but no evidence of slip was observed in the core. Associating all major core fracture and fault zones is of interest especially from the second phase of drilling, with previously published geophysical data that constrains fault locations within some depths of the borehole, but not the entire retrieved core. Potassium feldspar fracture fill increases in intensity in the vicinity of fault zones that are captured in the core; lack of slip direction indicators as well as slip surfaces; and inclusion of many < .5mm thick continuous semi-vertical fractures throughout the core were observed.