Sunday, December 7, 2014

DFDP-2 @ 850 m, mylonite, borehole shape

Rupert Sutherland, GNS Science and Victoria University of Wellington
John Townend, Victoria University of Wellington
Virginia Toy, University of Otago

Are we there yet?  They were born when we started and are growing up fast. 5/12/14.  Photo R. Sutherland.

 It was a slow start yesterday as the reconditioned drill reamed its way into the hole, but by this morning we were making new hole and producing fresh rock cuttings. We are now making about 3 m of new hole every hour and passed 850 m at lunch.

The good news is that analysis of rock cuttings is indicating we are not far from the fault. We know this from fragments of mylonite, which is a rock formed at temperatures of more than 300°C by the smearing out and recrystallization of quartz grains.

In the meantime, geophysical logging of the borehole (see last blog) and analysis of data is producing interesting results. 

The borehole is J-shaped, with the bottom deviated 40° from vertical (animated figure). This places the bottom of the hole below the Whataroa River about 200 m northwest of the drill rig. This is exactly what we had hoped would happen: the deviation is taking us directly towards the fault. The onset of mylonite rocks is right in line with predictions that we should be getting close.

Borehole geometry to 825 m.  R. Sutherland

We have been able to make a very detailed analysis of the inside surface of the borehole (example shown in animated figure). We have discovered that the layers in the rock consistently dip southeast at about 60°. This is a little steeper than predicted and a slight concern, as it could mean that the fault is deeper than predicted. The mylonite cuttings are giving us a clue that we are close, but we will soon find out where the fault is.

Borehole surface over a 1.5 m interval.  J. Townend

Funded by: the International Continental Scientific Drilling Program (ICDP); the Royal Society of New Zealand Marsden Fund; GNS Science; Victoria University of Wellington; University of Otago; and governments of NZ (MBIE), UK (NERC), & USA (NSF).


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