Oriented microfractures in Cajon Pass Drill Cores: Stress field near the San Andreas Fault

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Abstract
Oriented cores recovered from the Cajon Pass deep drill hole near the San Andreas fault in southern California were examined under both optical and electronic microscopes to study the distributrion and orientation of microfractures in the rocks. The objective was to determine whether there may be any systematic relationship between the microfractures in the rocks and the stress field in this area and to test the hypothesis that the observed shear wave birefringence can be caused by in situ microfractures. Three sets of microfractures, all dipping subvertically, appear in the rock cores studied. The oldest set (set 2) consists of mineralized microfractures with an orientation of N40°–50°E. A younger set (set 3), less prominent in appearance, consists of mineralized microfractures with an orientation of N68°–76°E. The most recent set (set 1) consists of open microfractures with fresh walls, with orientation of N14°–28°W. The two mineralized sets of microfractures were most likely formed in situ. The last set of microfractures with fresh walls was probably formed during core extraction and stress release. Based on elementary consideration in fracture mechanics, the second and the last sets (sets 3 and 1, respectively) of microfractures are consistent with a mean orientation of the present state of stress at the Cajon Pass drilling site, with a mean direction of the maximum horizontal stress at about N70°E, as determined from recent analyses of stress‐induced well bore breakouts, hydraulic fractures, and shear wave birefringence. The oldest set of microfractures (set 2) could have formed during an earlier strain event when the crystalline basement beneath the Cajon Pass area experienced an earlier strain episode when the basement block occupied a more northerly position during which the maximum horizontal stress was oriented at about N45°E. On the other hand, it could also have formed under the same state of stress as the present one, but the basement block could have rotated counterclockwise.