Oriented microfractures in Cajon Pass drill cores: Stress field near the San Andreas Fault

Conference · Tue Jul 10 04:00:00 EDT 1990 · Journal of Geophysical Research; (United States)

OSTI ID:5456107

Wang, Chiyuen; Sun, Yan ^[1]

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1. Univ. of California, Berkeley (USA)

Oriented cores were examined under both optical and electron microscopes to study the distribution and orientation of microfractures in the rocks. The objective was to determine whether there may be any systemic 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{degree}-50{degree}E. A younger set (set 3), less prominent in appearance, consists of mineralized microfractures with an orientation of N68{degree}-76{degree}E. The most recent set (set 1) consists of open microfractures with fresh walls, with orientation of N14{degree}-28{degree}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. 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{degree}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.

• OSTI ID: 5456107
• Report Number(s): CONF-880531--
• Conference Information: Journal Name: Journal of Geophysical Research; (United States) Journal Volume: 95:B7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
BASEMENT ROCK
BIREFRINGENCE
BOREHOLES
CALIFORNIA
CAVITIES
DEVELOPED COUNTRIES
DISTRIBUTION
DRILL CORES
DRILLING
ELECTRON MICROSCOPY
FEDERAL REGION IX
FRACTURE MECHANICS
GEOLOGIC FAULTS
GEOLOGIC FRACTURES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
MECHANICS
MICROSCOPY
NORTH AMERICA
OPTICAL MICROSCOPY
ORIENTATION
REFRACTION
SEISMIC S WAVES
SEISMIC WAVES
STRESS ANALYSIS
USA