expedition 314 summary expedition 314: sept - nov, 2007 logging-while-drilling dedicated holes at 5...
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Expedition 314 SummaryExpedition 314: Sept - Nov, 2007
Logging-While-Drilling dedicated holes at 5 sites
Philippine Sea platemegasplay
top of subducting basement
accretionary prism
forearc basin
Logging While DrillingLogging While Drilling
Bulk density/Porosity/Caliper
Vp by check shot
Data transfer/annular P
Sonic velocity
Resistivity image
LWD (Logging-While-Drilling)
Drill bit
1-2 rot/sec
Site Overview
Philippine Sea platemegasplay
top of subducting basement
accretionary prism
forearc basin
Site C0002: Kumano Forearc Basin and Underlying “inner wedge”
Site C0002 LWD log summary
Checkshot data for C0002
Site Overview
Philippine Sea platemegasplay
top of subducting basement
accretionary prism
forearc basin
Sites C0001 and C0004: Mega-splay thrust sheet
C0001 Log Summary
Borehole Breakouts:Indicators of Principal Stress Orientations
LWD Resistivity Image log
Walls of the borehole fracture or “break out” perpendicular to the direction of horizontal maximum compressional stress SHmax
SHmax
Shmin
Site C000
1Synoptic view: large horizontal exaggeration
Breakouts well-developed from 60-550 mbsf
Deeper interval has rare breakouts, abundant tensile fractures
Shmax ~ 336˚normal to thrust strike suggests
thrust stress state
… but convergence obliquity must be
taken up elsewhereConvergence at 310˚
- 315˚
SHmax
Shmin
Shmin
SHmax
Site C0003 - stuck and abandoned LWD string
Sites C0001 and C0004: Mega-splay thrust sheet
Site C0004
Thrust fault zone at 250-292 mbsf
Breakouts cross this interval with no significant change in azimuth
Site C0004
SHmax ~ 320˚
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C0004 main splay fault interval
Site Overview
Philippine Sea platemegasplay
top of subducting basement
accretionary prism
forearc basin
Site C0006 - toe of wedge
Site C0006
SHmax ~ 330˚
Site Overview
Philippine Sea platemegasplay
top of subducting basement
accretionary prism
forearc basin
Site C0002: Kumano Forearc Basin and Underlying “inner wedge”
Site C0002
Breakouts are well-developed to 1400 m depth
Mean azimuth = 134°
C0001 C0004 C0006C0002
forearc basin prism toe
These observations are consistent with:
• a compressional and/or transpressional stress state in the outer wedge
SHmax = 1
• an extensional stress state in the Kumano basin AND the underlying inner wedge, at least to 1400 mbsf.
SHmax = 2
This does not require a rotation of principal stress axes, only a change in their relative magnitude.
Upper 950 m:forearc basin
Lower 450 m: prism
Faulting Stress Regimes
Thrust
Stress Regimes from BreakoutsStress Regimes from Breakouts
• Horizontal principal stress axis change is sharp, lies between Sites C0001 and C0002
• Consistent with change in relative magnitude of principal stresses
extensional compressional +/- strike-slip
Future Work
1. Constrain stress magnitudes through strength testing of core samples
2. Drill deeper at C0002 (forearc basin site)
Will stress axes flip/rotate with depth?