expedition 314 summary expedition 314: sept - nov, 2007 logging-while-drilling dedicated holes at 5...

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?